I've played a number of games both solo and with my roommate... And we've both noticed the same major issues. The AI in Sins is down right stupid... Several very simple "facts" are present in all actions by the AI.
The AI does two things in this game...
1) Runs from larger fleets... and continues to run even if the AI is down to a single home planet... it will not defend... Just run. This is not "strategy" or "tactics" its just lazy programming... The AI should stand and fight... and retreat damaged ships to friendly systems as needed... Running the entire fleet away from system to system does nothing but annoy the players in the game.
2) Massive siege fleets. While these "can" be prevents or destroyed with a few ships and system defense buildings... its a lame strat. Siege craft should be VERY weak and easily destroyed if not defended. Its gotten to the point we don't even bother playing Sins anymore as we're fed up with trying to defend EVERY single planet in our "empire" against the AI (or another players) siege fleets.
Siege craft need to be weakened (not the damage... but the HP/Armor/Shields). These units should be nearly defenseless and vulnerable to direct attack. They should be used as "support" units brought in after the main fight is over.. to 'siege' the planet... Not as a primary attack force that suicides a planent.. destroying the ground structures and then running away.
I recall reading that the AI in this game was "too easy" and I find it sad that the only fix for this was to create hit-n-run siege fleets to annoy players endlessly. If they AI put as much resources into building normal fleets as it does into siege craft... I might actually have problems destroying hard AIs!