I would say Project CARS is better than Forza Motorsports 6 as a racing sim, though FM6 looks prettier, (comparing with a wheel experience on the free version of FM6).
Project CARS loses it's appeal quickly to racing fans who are into simcades or arcade racing, because it only does the driving part well. It's solo career and AI are more like an afterthought. Unlike the player, AI is unaffected by weather conditions, and terrain when they go off track, only simply coded to go slightly slower. They do this with slick tyres while the player has to pit stop for grip tyres in heavy rain or they can forget about racing. They pretty much still go full speed while the player struggles.
Also, while the AI avoids each other, they think they can simply go through the player to keep their programmed racing line most of the time. And contact is fatal to the player. Any contact cause your car to go straight, and steering and brakes just fails, while AI recovers straightaway, which leads to frustration with the AI.
However, the cars, tracks and the driving is very enjoyable (again from a wheel experience). Some people play racing sims to relax and de-stress, and I find this to be true of PC for me. That said, the traction on many cars seem to be off for me. I tend to play racing games on hardcore (without assists) but this game perhaps due to it's intended 'realism', has me spinning out on turns much too easily. And because of that and the AI, I often find myself resorting to the chase cam instead my preferred cockpit view for racing. At the minimum, I require ABS to be enabled for my habitual late braking, if only to beat the cheating AI.
PC can still be enjoyable for arcade racing gamers since AI 'skill' is adjustable (how fast or slow they take corners), and it plays well with the controller with the 3 assists enabled. But it still does not have the sense of progression/satisfaction players look for in arcade racing, though it seems to satisfy driving purists well enough.