I've been playing against Hard and Nightmare AIs because the Normal AI is very weak. To sum up what I'm talking about, here are the stats at the end of a 1v1 game that I recently won. I was playing on the Prison map with Oak; the AI was Erebus. Pretty much every 1v1 I play against a Nightmare AI ends up like this, though.
Level: Me: 18; AI: 25
Kills: Me: 0; AI: 6
Deaths: Me: 7; AI: 3
Assists: Me: 1; AI: 0
Damage: Me: 205820; AI: 585955
Flags: Me: 15; AI: 15
Buildings: Me: 4; AI: 6
Grunts: Me: 227; AI: 430
Gold: Me: 34025; AI: 72728
I was able to win only because I held the gold mine for virtually the entire game, and I spent all my money on creep upgrades. By the end of the game, the AI could kill me in about 15 seconds if I didn't run (that was an improvement; he hit the level cap when I was at level 14, and then he could kill me in 5-10 seconds -- just over the duration of his stun). I don't know how much of a bonus the AI is getting, but it's clearly getting a huge bonus to gold (6 kills accounts for maybe 10,000 gold, so he got at least double my gold income despite not even owning the gold mine). I believe it's also getting a bonus to XP: I can stand there and do nothing but kill creeps, and watch the AI level past me. And at the beginning of the game, when we were both level 1, the AI *regenerated health faster than I could damage it*.
Does anyone know what bonuses the AI gets? Also, as a feature suggestion, it would be nice to be able to set the AI difficulty and the handicapping separately, and to to choose which handicaps it gets. The XP bonus seems to be the most problematic: if I take the 10% +XP favor item, I can more or less keep up with it, so the game is on a much more level playing field and much more satisfying.