Definitely not Orcs; they also have some uneven tiers (Centaurs, Shamans, Wyverns, Cyclops).

In my opinion, Centaurs are only slightly different, relatively speaking.
Shamans: Earth Shamans are better without Blood Magic, while Air Shamans are better with Blood Magic due to Chain Lightning; it's quite clear.
Wyverns: I agree.
Cyclops: Free Cyclops are indispensable during sieges and attack a large area; Blood Cyclops have slightly better stats and are more predictable (the enemy won't catch you off guard with an area attack on their own units; with Free Cyclops, you have to be careful, which is probably especially relevant against humans), and Blood Cyclops also have an aura. In the end, they aren't that unbalanced.

Humans are relatively balanced; the only issue is with Archers, and sometimes with Monks.

In my opinion, the imbalance of Archers covers the imbalances of almost all other factions (except perhaps Elves and Terrans, which might stand out in this regard). In addition to what I mentioned, there are also Griffins. Everyone takes Royal Griffins. In general, the balance of the Order is very far from ideal.