At the highest levels the AI has twice as many units as the human, better promoted. It's too hard to build, and rebuild, enough horsemen or hunters to take out waves of attack stacks.
I agree about stack attacking, but only catapults are at all early enough.
At the highest level of difficulty, all AIs are overpowered by definition. The clan will have 4X as many units as you (assuming your empires are of the same size, more often than not they will have a much larger empire and therefore even more units), and they will all probably have combat III and up due to successful wars because of the sheer number of units they can field. The hippus will have waves of high withdrawal units that will wear you down while taking minimum losses themselves. Every non-elven AI will spam catapults and assassins to high heaven. Then you get the mercurians who get ridiculously strong units for every good unit you kill. Actually, the mercurians are the largest thorn in my side in my deity games. The thesis "Pyre zombies are hard on deity" is redundant. Everything is hard on deity, and it is supposed to be hard. That is a good thing. What is the point of a deity difficulty that is easy?
Pyre zombies are not "easily" countered. They are possible to fight, and far harder to do so than normal axemen.
The have the same advantages as axemen with their strength, butin addition, they explode on death, and they have no building requirement, making them spammmable.
Lets assume a Sheaim Civ and a completely vanilla(no special traits, uu's, buildings) civ of comparable size and technological parity. The sheaim, when they attack, will not be able to send 100% of their forces to attack you. This is for obvious reasons, but I will elaborate on them anyway: general civ defense, barbs, fear of counter attack fear of other AI's, Faeryl Viconia, etc. Therefore, assuming parity, you will have a larger defending force by definition. Now, if the player controlling the vanilla civ made good choices knowing that the Sheaim are their neighbors, they will have a relatively easy time repelling said opponent. They would have been able to amass some withdrawal horseman, some siege, mobillity promoted axemen, [insert any unit with 2 moves or more], and possibly some assasins/priests (these are a bit more advanced, but it also takes time to build a large horde of pz's, which may or may not give you enough time for said units).
In the above situation, the vanilla civ in question should be able to repel the pz's with relative ease, because it will have a moderate advantage in numbers. Withdrawal/collateral/marksman/stackbusting spells soften up the pz's, mobility units pick off pz's one by one. Done.
Now, there may be a problem if the Sheaim have a larger or more productive empire or a technological edge. If this is the case, then the player controlling the vanilla civ will be out manned and
should loose. Furthermore, if playing on a high difficulty, the settings were designed to create a situation where the player
should loose. Working as intended.
So, assuming parity (which is the only real context possible for
balance discussions), the defender will always have a moderate advantage in quantity of units and the ability to reinforce his own army quickly which combined with an intelligent combination of units, can easily counter pyre zombies in moderate size. Now a stack of 100+ pyre Zombies may be another matter entirely, but by the time the ai can field that, the player had the opportunity to get life II or toher appropriate counters.