The UNSC AI is far weaker than the Covenant's because the former's ships are based around alpha damage and being deployed in groups. This contrasts entirely with how the AI in Sins works where it is designed around protracted attrition warfare, meaning that it will send ships in piecemeal. Covenant ships, given their superior durability and DPS focused weaponry, will tend to function far better with how the AI operates.
This thread is outdated; 1.90 came after this thread was made, and it is as of that version that LAA is enabled by default, and thus people should not do it manually. I'm locking this thread to avoid confusion; if someone is having issues still please make a new thread.