I cannot attest to the durability of 18F series but I've stressed quite a lot of 16F and 10F series, intentionally. Overvoltage, undervoltage, back-emf, shorting all outputs, oscillating power supply. Except one which did not want to be programmed anymore (I have not looked into it, it might have been a connection issue) not a single part died. Granted, this is not testing, just anecdotal evidence.
Last week I put 9V through an 18F4520(?) for quite some time until I noticed the current draw. I was debugging a prototype board a friend had made and the regulators were not fitted properly. The board also had some shorts ('by design') and in the end a peripheral pin died. I'm not exactly sure when it died, but by the time we got to write code for it we had to fix about 20 something board issues. The offended pin was wired to a 74LVC4245A octal chip.
Actually, if someone has the chips and the inclination do to so, I would love to see a reliability compare between architectures: one more thing to add to the PIC vs AVR vs ARM vs ... topic. I would love to do it to TI valueline chips but I only have a handful of them.