When it comes to the type of car mods disallowed here on RD - mainly Forza ripped cars, there are absolutely more junk cars than there are hidden gems. But! That means there are some hidden gems. It's really a hit or miss thing. I am willing to bet there are some Facebook Mods (the generic name used for these ripped cars because that's where you find them) that I think are pretty decent and Michael Hornbuckle are awful and vice-versa.
Since it usually doesn't take much time to discover if the car is A) realistic at all and B) a decent enough representation for the make & model car they're trying to simulate - if I find a good car I give it 2-4 stars in CM. If the 3d model is good, but the physics are just not enjoyable and it's beyond my skill to fix, I assign 1 star in CM and I only put AI drivers in those cars. At least I can always have a fresh trackday or race with different cars.
Oh, almost forgot -- this doesn't happen very often, but there are some FB modders who produce their own custom sounds for cars. Believe it or not, I've come across a few cars that had a good set of sounds but drove like crap. So, I kept the car for the AI to drive, plus I used the sounds from that car on other AC projects, like fixing old mods.