Hard and Nightmare AIs get gold and stats cheats. To add insult to injury, your own AI allies in Tournament remain on Normal.
In both Hard/Nightmare it's important to either 1) Win quickly, or 2) Deny the enemy team the artifact shop (those flag locks are awesome). Exile is a very bad map for Tournament since the artifact shop is in each team's base, so you have to win that one quickly which is tricky if it's a Conquest.
If the enemy starts getting artifacts, they'll just farm your AI allies for xp and even more gold and it's bad mojo.