Let’s look at the Konnik, the unique unit of the Bulgarians. For our comparison let us use two similar units, both in price and in function, the Knight and the Boyar, one regular unit and one unique unit, both considered to be well balanced.
The Elite Konnik compared to the Paladin lacks 40 HP and 1 pierce armor, to compensate for that the Konnik spawns a unit comparable to the longswordsman upon death. They can be made from Kreposts which is an advantage over other unique units but considerable worse than only requiring stables like the Paladin. The Boyar by comparison lacks 30HP but receives +4 melee armor compared to the Paladin.
What this means in practice is that a FU Paladin can take 60 hits from a FU Arbalest, the Boyar 50 and the Konnik 30 + an additional 10 from the dismounted Konnik. In Cavalry combat the Konnik is equal to the Paladin, the winner coming down to who hits first, while the Boyar wins comfortably with about a third of his health left.
So far we can see the use of the Boyar, an excellent melee fighter offset by a slightly stronger vulnerability to range and having to be created at castles. The balance is good since overall it’s preferable over the Paladin but harder to amass.
The Konnik on the other hand is terrible against ranged units and equal in melee. There is only one situation where the Konnik is better than the Paladin and that is against Halberdiers. The problem is that you never want to use the Konnik against Halberdiers since it’s still a terrible trade and you’ll be left with a nigh useless dismounted Konnik. But that is not all, the situation for the Konnik is even worse since you have to research Stirrups to get the attack speed to be the same as other cavalry and you also have to research infantry armor for the dismounted Konnik. This is not trivial, you will require alot more resources and time compared to the player who goes with Paladin and you end up with a unit that’s overall less desirable than it.
I have discussed here the situation of the FU Konnik and one might object that it’s a unit that excels in the Castle Age, because it can be created from the Krepost and is thus more readily available than say the Boyar. To that I will simply say that the Konnik is not a good Castle Age unit since you would have to build a Castle anyway in order to research Stirrups and without it the Konnik is just as bad in it’s non-elite version, having only the slightest advantage in a knight-fight and is considerably worse against crossbows.
The Konnik is therefore a weak unit and was sadly over-nerfed in previous patches and there needs to be a change either in the power of the unit or in the cost and time it takes to upgrade it. If the Konnik is supposed to be strong and quicker to amass in the Castle Age it cannot be required to have tech from both a Stable and a Castle. If it’s meant to be strong in the Imperial Age then Konnik needs to be stronger than the Paladin just as the Boyar is and even more since it requires more tech investment than the Boyar.