I don't know what's wrong with the AI, but I think it's a bug. A really bad bug.
Some examples in my current game in a Gigantic Galaxy, 7 AIs, Suicidal settings.
1) For some reason, most of the Iconian population on their planets is 5 million. YES, you got that right. MILLIONS not Billions. Invasion is a breeze since I land 1 BILLION troops at a time. What's wrong with the Iconians?
2) The Terrans have spreaded like a plague and have influence on over half the galaxy now. BUT they have done nothing into weapons research! I have just invaded half their territory and have met with zero military resistance!
These bugs really kills the game. Thanks alot of wasting my time Stardock!!!