They have to be light to focus quickly--no way the mass of a MF lens could be accelerated that fast. I've found mine to be remarkably tough but if you're sadistic or accident-prone, they will break--big surprise.
The 35/2AF is notorious for oily blades--no news there. If you're buying hi-mileage, used lenses, then the chance of problems just increases. BTW, MF Nikkors aren't immune to troubles either: dried-out helicoids, sloppy focus, impact damage, loose aperture detents. They all wear out and/or break.