Am playing Nemesis of the Roman Empire, an old RTS that never got much advertising, and I discovered something crazy. Of the three AI options, apparently the medium AI (which still beat me quite a few times before I learned to beat it reliably) is intentionally gimped. Like super badly. There's a once per game ability called tribute that that after a minute and a half (with a globally visible timer) gathers a large amount of gold in each settlement you have which you can then send back to your strongholds. I assumed the AI just didn't know how to use it, no apparently the hard AI can. The particular faction they were playing as (Iberians) has mostly units that are highly specialized or even potentially detrimental. The medium AI just can't handle that, but the hard AI grabbed the couple of units that are generically useful and then formed the rest of their army out of neutral units or units from other factions.
The most incredible thing is that the medium AI apparently only sends out one army at a time for each task it wants to do. I learned this because the hard AI has no such restriction and crushed a 150 strong, fairly elite army with like 300 units of its own.
I was playing on a huge map with 6 medium AIs and one hard AIs. Apparently the hard AI not only buzzsawed through the medium ones just as well as I did, it actually took more land than me. The only thing I will say in my defense is that once your nation gets to be a certain size the game can be really hard to control so of course the AI will do well at that. Not sure if it cheats on top of all that but its so weird to me that a game from 2003 would intentionally make its mid level AI less intelligent.