AI is actually pretty damn impressive in the rFactor engine. Not perfect by a long shot but still impressive. Reasons for AI being screwed up in most Mods (and even in some of the official cars) is due to crappy AI setups and bad AI racing lines.
Also, AI doesn't aggressively block their racing line so it's easy to "abuse" this by divebombing. However, having said that; treating the AI like a HUMAN player with strict rules, like in iRacing where you get severely penalized for reckless driving, you'll suddenly notice the AI being quite damn impressive if you give it some space. This can be easily witnessed in Simbins GTR Evolution / Race On series where they have paid some good attention to the AI behaviour. It's by far the best offline driving experience I've had since Grandprix Legends.
My only gripe is that the AI isn't very fast, especially with tin tops.. in open wheelers they seem to be better (or then I'm just much worse!). I can easily beat the AI on 100% difficulty and I'm not even that fast of a driver.
Cheers!
bManic