Today, I came across Ivaylo. He is an Elite Konnik hero unit. Now with those following the balance changes will know that the Bulgarian Cavalry has been changed thrice. Usually units in AoE2 are made by copying data of similar units. A very prominent example of this is Cataphracts, Konniks, Keshiks, Leitis and Boyars all have 0 attack bonus against archers, while Huszars and War Elephants do not. This has a direct implication that they have been created by copying the data of Knight (Knights since AoK had this attribute since Persian Knights were made to have +2 attack against archers). Now in RotR, Knights were made to have 0 attack vs buildings and standard buildings (for Manipur Cavalry). Sometimes, these changes are manually copied into other similar and hero units. Many times, they are not.
Now back to the topic of Ivaylo. Upon release, Konniks attacked every 1.9s and 2s (latter for dismounted). This was also the case with Ivaylo unit. Stirrups explicitly affected Konniks and Light Cavalry line. So no luck for the hero unit. Next, the attack rate of Konniks were set at 2.4s. Ivaylo unit and Stirrups remained unchanged. After Stirrups, mounted Konniks attacked every 1.8s. Last change was removal of Paladin and Stirrups technology being made to affect all Cavalry units (in standard game it meant it also affected Knights and Cavaliers). This meant that Ivaylo with base attack speed 1.9s, attack every 1.425 seconds, which is fasted in the cavalry branch, 2nd fastest for heroes and tied with Samurai and Japanese Condotierri. Now insane stats on a single unit do not break the game always, that too an important hero unit.
But sometimes, it makes harder when base units are buffed or are affected by specifically programmed bonuses. The Boyar unit (both forms) received +1 pierce armor and -0.1 speed. The Vlad Dracula hero unit obviously did not receive the changes. The Mongol Mangudai has normal +4 pierce armor after all upgrades while the Genghis Khan hero unit is stuck at +2 because it doesn’t benefit from Parthian Tactics (neither of the two effects). I haven’t checked the Kipchak and Steppe Lancer hero units. Another honorable mention is Theodoric the Goth, which was based on Castle Age Huskarl in AoK, which then only had 4 base pierce armor. The lack of Plate Mail armor for Goths also makes it rough for this unit. Of all the units, I am most stoked on the Parthian Tactics one, since armor can be important for Cavalry Archers, especially when civilization missed Ring Archer Armor.
Another change which I am not mentioning, because it is intentional is change in Battle Elephants. Usually hero units are in sprites of Elite Battle Elephants. This unit received -2 nerf, but extra attack on hero units is normal. Another change was less trample. That should not affect hero units because all melee elephant hero units deal full trample damage in their blast radius.