Poking around my shipdata dump, which may be getting a bit out of date now that I think of it, the only big thing I see that might effect the balance is missile guard posts, or possibly beam guardians. They both have an 8x bonus vs. ultra-light. Just about everything else that is good vs. ultra-light hulls both sides get access to. Missile guard posts are pretty common though. Just guessing, but maybe that's what seems to chew up human raid starships so quickly while the AI versions last a while.