Single player offline has fewer bugs than online, but you are still likely to encounter some weirdness with the AI's behavior. Inconsistent strength, inconsistent wet/dry pace, inconsistent pit and tire strategy, bad bugs with rolling starts (both in manual and auto driver during pre-start), pretty poor standing start behavior in most classes (too slow, you're usually mid pack or much better at T1 starting from the rear in standing start mode).
But compared to GTS, AI pace can be set MUCH higher. Enough to challenge all but the aliens.
You'll enjoy the game's offline mode until you run through it. One of the most common requests, custom offline championships so you can set your own challenges, cars, weather, tracks etc., has been ignored by SMS since the start of PC1. You can set anything up as a one time race (although you get no choice in opponents other than by group or one makes, so it's tough to dial in exact lineups), but championships, multi-race series, like the Career Mode does, you are out of luck.
And no, the bug-list is still War and Peace length. Quite a few biggies still exist, SMS have given up on their previously comprehensive patch notes so it's getting harder and harder to track down what's fixed, what's placebo, and what's newly added bugs (they always seem to add a few every patch!). Check out Asturbo's post on the PCARS2 forum...
Me, I don't blame crowdfunding. I blame SMS biting off more than they could chew, and then going bye-bye from their own forum and any significant player interaction while they work on the next game (some sort of mobile game is next, apparently). Looking at the unfixed buglist from PC1 to PC2, it doesn't bode well for PC3. Me, I'd rather fewer new features and the ones they already have working perfectly... But that seems to be an unpopular idea round here..!
Bottom line... don't trust ANYTHING. Don't trust that the setup you just practiced with will still be on the car as you go out to qualy or race. Don't trust that setting tire strategy to Automatic will work. Don't trust that your gear ratios won't mysteriously change in between sessions. Don't trust that the AI won't slam into you (or pass before green) during rolling starts. Don't trust anything automatic, and you will have a far less frustrating time racing the AI...