In general the eight original factions have a standard formula for their combat improvement cards. The cards will buff a few units with +15% attack, Hit points or both. And usually come in a set of three. Like infantry attack, infantry hit points and infantry combat for a total boost of +30% attack and hit points. This is the general design, but there are exceptions and variations on this. Also with the release of the definitive edition, patches and faction reworks there were some changes to some of these old cards. But with that we can see that not every faction is treated equally (yet) in this way. Here I try to go over each of the original eight and point out inconsistencies and were cards might have changed from the times of the Asian dynasties. I’m purely looking at raw combat stats so a card that increases movement or damage bonusses versus certain units will be left out of this.
Do note I’m skipping all naval cards please forgive me.
British:
The British have two sets of buff cards, one for grenadiers and musketeers and the other for cavalry. The Musk/Gren cards are quite standard but do contain a TEAM card that will not only buff musketeers and grenadiers but also similar units that share the overall type, like soldado and Ashigaru and also buff mercenaries and native units. This is standard for the TEAM cards and each civ has at least one of them.
There is something going on with the cavalry cards though and these have changed since the Asian Dynasties. In the olden days these cards would buff the Hussar and Dragoon of the British and nothing more. However at some point in the DE the cards have been upgraded to buff all cavalry, any other type of cavalry now obtained by the British will have access to the full +30% attack and hit points that formerly only their Hussar and Dragoon could get. I do not know at which point this change was made, but it sure is significant. You might want to let your British ally take that native post if it contains cavalry units.
The cavalry combat card that gives +15% atk and hp is an interesting card. For it is present in all but 1 of the original civs. For most of them it always worked as it does now, but for the British is also has been upgraded as it formerly only upgraded Hussar and Dragoon but now upgrades all cavalry as it does for most civs (1 exception remains).
There are two more combat buff cards for the British, Improved Grenades for the Grenadiers and Ranger combat for the Rangers. Fine cards overall if you want to use these unit and nothing much to tell about them other than that the Ranger card is hidden underneath Siege Archery until you unlock Rangers by another card. Since British were the first of the civs hopefully the other texts can be shorter.
The British have a total of eight cards that buff their unit’s raw stats.
Dutch
The Dutch only have five cards that buff their unit’s stats but they are efficient and buff their most important units. There is a set of three cards for their infantry and this buff their Halberdiers, Pikemen and skirmishers. The TEAM infantry hit points card buff all infantrys hit points.
Then they have the standard cavalry hit points card and an interesting TEAM Coehorn Mortars card for Mortars and mortar like units in other civs. Overall a solid suit of 5 cards(unless I missed some) and also one of the smaller sets.
French
The French have the second largest set of buff cards with a grand total of nine cards with three of them being TEAM cards. French are team players for sure.
They have a set of three cards for their cavalry with hand cavalry hit points and TEAM hand cavalry attack. It does leave the ranged cavalry out but the standard Cavalry combat card again is available.
For infantry there is the TEAM ranged infantry attack card which almost everyone will be happy to see. Then there is a Long-range infantry hit points card that affects the French crossbows and skirmishers (and revolutionary sharpshooter). More interestingly there is Wilderness Warfare a strong 20% hit points card for skirmishers, Coureur de Bois and native warriors. The French continue their native theme with Native Warrior combat and a TEAM native warriors card(French Auxiliaries Combat) giving native warriors a potential +25/+45 in stats. And that’s without potential buffs from the Cavalry combat and other two TEAM card that all can benefit natives(or mercenaries).
And on top of that all there is the Gribeauval System card a massive 25% increase to the French artillery (20% for heavy cannon).
Germans
The Germans have the largest set of raw buff cards with ten in total. They lead with a standard 30/30 set of three cards for hand infantry, but do note it only buffs pikemen and Doppelsoldner. Natives and mercenaries are unaffected by these cards as they are by most of the Germans cards. Doppelsoldner can buff their stats further by a massive 50% at the cost of 33% movement speed by sending the Solingen Steel card creating beastly yet slow melee warriors and dangerous siege troopers.
Then there is a single card for their crossbows and skirmishers hidden in age IV giving them +15% hit points.
German cavalry has the standard set of three cards to buff them, but besides the TEAM Cavalry Attack card these cards only affect Uhlans and War Wagons. The Germans are the only civ with access to the cavalry combat card in which the card doesn’t improve ALL cavalry. Probably a side product of being coded to deliver those four Uhlans, missing out on the standard version of cavalry combat this way.
There is a fourth card for the German cavalry but the Lipizzaner cavalry only upgrades Uhlans leaving your other cavalry stranded on sub-par mounts.
Finally there is Improved Mercenaries 20% stronger mercenaries in age III, but note that the only other card that can potentially improve those mercenaries is your TEAM cavalry attack(or your allies TEAM cards ofc).
Germans have a lot of cards but are quite specialized in which units they buff, luckily you’ll have plenty of Uhlans around.
Ottomans
Ottomans have a small set of five cards, three of which are actually for artillery. They are quite interesting with Light artillery hit points buffing artillery troopers and grenade troopers and a further artillery hit points card that does the same but also includes artillery. And then a TEAM artillery attack card that boosts their damage as well. None of these cards mention specific units so the rare native and mercenaries with these tags get to enjoy the boost as well.
Then there is a solid 20% for Janissaries with Janissary Combat in age III, which has been buffed in the DE to also affect Nizam Fusiliers.
And finally there is Cavalry Combat again, a solid card that seems to be a standard in European card line ups (F for the Germans).
Portuguese
Portuguese are the only ones of the original eight to not have Cavalry combat, instead they have the Dragoon combat card, it improves you Dragoon by 20% and your hussar by 15%. So it could be a bit better than cavalry combat unless you are also using alternative cavalry like natives or mercenaries.
There is also a set of three card for Gunpowder infantry. Combat and attack will only improve your musketeers and cassadores, and the TEAM gunpowder infantry hit points will improve all gunpowder troopers hit points.
Finally there is Artillery Hit points a slightly weaker version of the Ottoman’s card that doesn’t include artillery troopers but still buffs grenade troopers and artillery.
Overall a reasonable set of five cards but I do wonder if a regular cavalry combat card wouldn’t be better than Dragoon combat.
Russians
The Russians really are the odd one out when it comes to these buff cards the only “normal” card they have is the Cavalry combat card which is solid as usual.
TEAM Kamchatka Expeditions boosts their cavalry hit points by a further 10% and includes a LoS bonus as well.
And their Cossacks and Oprichniki receive a further +15/+15 from the Boyars card which also happens to buff their Strelets by the same amount.
The Strelets can receive another 20% health from Strelet Combat and another 10% health from Strelet Horde, which also spawn a bundle of them for each Blockhouse you have or will build from that point on.
Their Musketeers can get a small boost of +10/+10 with the national redoubt card and that will also allow them to rebuild any forts you may have lost.
Then there is Unicorne Mortars which improves Mortar damage by 30% and also makes them cheaper and build faster.
Special shoutout to Ransack for making all Russian infantry a lot scarier vs buildings.
Spanish
Finally we come to the Spanish who have at first sight a rather bland set of cards. They have two set of three cards, the first of which is focused on hand infantry or rather archaic infantry. The names of the cards appear to have changed since Asian Dynasties and only the TEAM hand infantry attack card is still named so, it affects all hand infantry including natives and mercenaries again. Something that the two now named Archaic infantry hit points and archaic infantry combat didn’t use to do. Since they were originally only affecting Pikemen and Rodeleros. Nowadays they affect ALL hand infantry AND foot archers, a major buff to these cards.
A similar thing happened to the three cavalry cards they have. Formerly these affected only their Hussar and Lances, while now they affect all Hand cavalry. The Cavalry combat card even affects ALL cavalry now, in Asian dynasties it also would only affect hussar and lancer. It seems it has been brought in line with all other versions of cavalry combat(angry German noises).
If you look at the tooltips of these cards (besides cavalry combat) you can still see which units it used to boost as they will still state as the first line “Hussar and Lancer damage improved” for example. The names of the cards and effect (hand cavalry: All actions damage +15%) are changed correctly though. A similar thing can be seen on the British cavalry cards. Odd that it only happened on the two civs that were reworked, a sign of things to come? 
Finally the Spanish update came with Peninsular Guerrillas which improves ALL ranged infantry damage by 25% and gives skirmishers a little tracking ability.
I do want to mention Unction here briefly since the nerf probably has something to do with the Peninsular Guerrillas card. But in general all Spanish cards will affect any unit of their type you might build where that formerly wasn’t the case in Asian Dynasties. I do wonder at which point these changes were made, but I do recall Cavalry combat being added with the rework. Was the old version formerly removed in DE?
If I stupidly missed any cards be sure to point that out.