Simply, they own when the AI uses em, and suck balls when a player does.
Ok, step one is to identify why this is. How in the world can the AI wield ANY unit better than a human can?
It's more the fact that they are good at killing single really important targets. The AI doesn't really have any. Command stations and warp gates come close, but there are units that can kill those just as well, AND do other things.
I think the problem is that they can't kill anything military. They should be able to kill either turrets, mobile military, or both.
Here's my ideas for buffing them against the following targets:
Turrets - Buff the damage up so they can slaughter turrets (and anything else they manage to catch).
Mobile Military - Reduce HP and increase speed by the same amount (preferably like 40 speed. Obviously you'd cut their hp by a ton e.g. 2/3s). Also increase damage done, just not as much as you would with the above. Also decrease the damage they take when attacking. Possibly either make them cheaper or raise the cap on them too.
Both - Some middle ground between the above 2 ideas.
The first idea would give them a sort of "slow, if I catch you you die" style. Second would give them a "mad rusher" type style.
Personally I prefer the first, since it's simplier (less likely to cause balance problems) and avoids revamping the entire unit.