After a weekend of running 'quick races' in the Special Events, I don't think the AI are any better than most other racing game I've played... the only game I remember with good AI remains TOCCA2 (and that had issues on a couple of tracks - eg; Oulton Park on Hardest difficulty was a nightmare).
AI in AC...
They have a magical ability to accelerate just as quickly when they have 2 wheels on the kerb and 2 on the grass on corner exits... try that yourself and you're immediately in the barrier/gravel trap
They always have 'perfect' traction exiting slow/mid speed corners... and sometimes they seem to have better than perfect traction
AI cars also seem to have some sort of massive magnet sticking them to the track, which means they can punt you at will and not spin, where as if you as much as touch them, they are unaffected and you spin
They will do anything to avoid contact with each other (witness the swerving and braking hilarity during race starts and 1st couple of corners on lap 1), yet your car seems to be invisible
The main issues is, like most other racing games, they are entirely fixated with being on their 'racing line'... so once they are on it (after the usual 1st corner melee), they hardly ever try to overtake each other (or you), and if you try to overtake them, they will do absolutely anything to stay on the line - even if you're >90% past them, and they are on the grass on the outside, they will still keep their foot in to try and hold the line... this typically involves them punting you
&
Having said all of that, if you find the right level of AI difficulty, and you have the patience to re-start (many, many times) until you survive the 1st couple of corners in a position that gives you at least a chance of winning, the racing can be pretty good fun... the Hurracan ST at Vallelunga, the tuned RX7 at Imola and the Z4GT3 at Nurb GT were all events I had to put a good amount of effort in to to get 1st place (AI on hard).