Personally I think the laps:yellow flags ratio is a very good indicator of how consistent the driver is. All the drivers I've looked at seem to have ratios as you would expect. The guys who I know can do 50 laps of a track in a league race without crashing have very low ratios. Those drivers who I know are prone to mistakes have higher ratios.
Seems pretty accurate to me.
Also, letting people past you when under blue flag without having to slow to virtually a standstill while at the same time not compromising the passers lap is also a skill that you learn in LFS.
The GFC with it's dual class formula teaches you how to do this.