I don't think it's the elevation change that's the problem, I think the fact that 90% of the time a crest lands on a corner is the problem. If you're mapping out a track in the real world you wouldn't have every crest co-inside with a corner.
Don't get me wrong, there's a couple of corners at the Nurburgring that corner on a crest but it doesn't throw your car out as much as what the track creator seems to do. A lot of tracks just become un-drivable.