Not going to affect LFS one bit.
In my opinion iRacing is a very well balanced packet although arguably a little expensive compared to your average simcade. Fortunately or unfortunately it's the only sim that has the best fit for me.
Catching slides is hard (easier with a direct drive wheel) and I guess in this department iR is lacking a bit. Hard to say whether it's the tire model or the insufficient hardware dealing with the realistic tire model.
AC doesn't run high enough FPS in triplescreen, I have to to drop almost everything to low/off to afford AA and AF. In iRacing I'm using 4xAA/16xAF 5760x1200 with locked @83fps (i7-965 extreme and a R9 270x). In comparison AC looks horrid and the sounds are horrid also and I have to admit I don't like the physics/FFB either, I think it felt better in the first few releases...
rFactor is a slidefest on easymode especially with the highly praised Honda NSX.
LFS was my first love, the cars were actually affected by physics in realtime. Replays were excellent and online racing a good match. But it's showing its age. I guess the biggest minus was when they added more code to the cockpit view, started shaking and made it hard to look at the screen with more realistic FOV since your eyes tired. Haven't really been able to dial it out. Again something where iRacing shines. Also the track surface is lifeless, can't really compare to laser scanned tracks.