I've worked with Parker Kligermann, and he has said the main problem is at low speeds and with the initial slip angles (I'm guessing at any speed). On take-off in the stock cars, you should pretty much be able to floor it on new tires and it should stick. Maybe alittle bit of feathering on some tracks with the Cup car. The amount of grip at low speeds seems to be far off. This creates people wanting to slide the car through the slowest part of the corner in order to gain speed and get the car to rotate center -> off.
The initial slip angle (another term is probably more proper for this, I'm too tired to think of anything else) also seems to easy to fall into. That's why most say that iRacing has less grip than the real thing, while LFS has too much.
The bit that really makes me scratch my head is how in the world can people in iR be 1 second faster a lap on a <30 second oval than the real thing. People say iR has less grip, but then why is it 1 second faster?
There's so many "what ifs". We can only hope they can get it right, but showing the amount of time it takes a tire model to be built (iR, LFS, rF2), I doubt it'll be dream-come-true perfection right off the bat.