I started my experience with Galactic Civilizations II from Steam Ultimate version and I cannot complain. I have not noticed the flaws mentioned above.
How did the Korath get 500-power fleets "early" in the game?
I had encountered a bug in one of my early games.
I was not aware of it. I just thought I was a noob. Ai started to invade me with fleets of around 500 attack and maximum logistics. My Drengins countered them with Attack 200 but much more hp, but I was loosing ships faster that I could build them.
Then I noticed something weird. The newly colonized Arcean planet produced 130 research flasks. That made me thinking.
I checked their stats. Thay got sick bonuses to some of the abilities. Economy around 800% and Industry with Research around 500-700% (I don't remember the rest).
They got Nightmare Missiles at the time I just finished Harpoons III,
They were three races (Yor, Arcean and Thalan I think - not so sure now) allied together (all had that weird boosted stats). They wiped out the rest, so I was their only target.
I had to start over but now in every game I check my enemies statc from time to time.
It has not happened again for now.