Do factions get specific bonuses? One AI cavalry can take a city from one defending player cavalry or infantry supported by one infantry. But one player cavalry supported by two infantry and two artillery, failed two assaults and was destroyed on a third, against a city held by one AI cavalry supported by one infantry. On the other hand AI infantry seems to be somewhat easier to dislodge than player infantry. 

TLDR: AI cavalry is OP and player troops are half as good as theirs, with infantry being the only exception.

P.S. This occurred back to back, once when I was blue and once as red.