Quite big I guess. Look at the number of F1 mods for rF1. I don't think most of them are of decent quality.
But the other side of a coin is that people can sometimes criticize realistic car, because they do not understand it. I don't remember which year was it - 2011 perhaps, when you could see Hamilton struggling with his new McLaren all the time at beginning of season. Car was horrible to drive, sliding all the time. I don't suppose he had an option to complain about physics of simulator, because it was in reality
For me, realistic car in a simulation is not something that behaves the way I expect - it's something that behaves like the real car it's supposed to mimic. And cars do differ in reality a lot.
One of my colleagues is a champion of 2 series already, but when he participated in a Porsche Cup, he told us that car is difficult to drive and he wasn't as successful as in other series.
I can only imagine simracers complaining to ISI about Porsche being badly modeled if such thing happened to them
I usually first try to understand what's going on, before I criticize anything. The same goes for praising something.
Some people complain about C6, while I enjoy it a lot. Not because it's a Corvette, but because I can understand this car and it's balance - it makes perfect sense to me.
And don't let my avatar fool you - I'm not some kind of Corvette fan. It's just an avatar - got an idea for it when I saw Dark Knight poster