In my experience:
Losi: Just plain wears quick, and not really the stuff you would expect. The camber link balls, the steering screws and bushing collars, metal shock caps etc. On the plus side most of the parts are cheap (Except the shock caps, a bit expensivce for how quickly they wear, I would look for an alternative if I got another losi).
RC8: Intially seems to wear pretty bad, say in the first 1/2 gallon it overlly wears a lot, but then over the next 4 gallons does not seem to wear much at all. Kind of like it breaks in and then stays that way.
808: Durability and wear is pretty good, just have to replace the center cvd/pinion bearings fairly frequently.
MBX-6: Hasn't really been out long enough to tell, but historically mugen is the most durable and reliable car you can get and does not really wear parts out very quickly. Traditonally Mugen is built like a tank, not sure now however as they are the lightest car on the market and border minimum weight and that would usually mean there is a compromise somewhere?