Or does the AI suck for the other 3 races compared to the original 3?
Yes, the AI sucks for the 3 new races. When you play against them they are really easy to beat, No, not really I have found that they are all a good challenge. So I am not sure by what you mean when you say 'effective', different yes.
the pirates are even stronger than the original for the first wave
Yes, you can't ignore them for the first couple of waves, after you have built some defenses and research some additional frigates and improved weapons/shields/hulls they become a small irritation.