The AI just doesn't seem to be able to cope with Guerrilla tactics. Granted I only have played up to challenging, but if I shove engines on a ship, and zoom around fleets just blowing up starbases and capturing isolated planets, the AI doesn't seem to be able to deal with it. I can ignore their slow moving fleets and pick my own fights.
Does it learn to build faster ships on higher settings? I get the impression that if I set the AI higher I'll get out-teched and out-econned rather than outmanuvered.
Granted, even a good human player has a hard time dealing with guerilla tactics. No reason why the AI would be that much better at it. Having the AI hit my sensor ships is important in allowing it to get invasion fleets by, but doesn't really help it when I'm privateering and blowing up their starbases.
