I think a lot of the complaints about AI come from reviewers only going through the novice part of the main races. It took reviewers weeks to get through GT5 when it came out, but now it seems like they just got the game, went through a few races and modes and called it a day.
I also think a lot of the gripes about engine sounds is due to them leaving the sound setting to Living Room instead of bringing it up to small theatre or large theatre. Collisions noise suck no matter the sound setting though, so they're right about those.
Yeap... I guess so the beggining is a lot easy.
About the sound I have mixed experience... some few cars sounds crazy accurate (like the Galardo) but other sounds like a vaccum cleaner... so bad... I think PD needs to work in the sound for PS4 (seems like they have memory limitations in PS3).