As far as I know computer parts get killed by both high temps and high voltages. Plenty of stories out there about damaged or degrading CPUs/GPUs that were overclocked with heavily increased voltage and were never overheated (so the damage was done by the high voltage and not the temps). That was actually one of the reasons nvidia implemented all that weird "boost" tech for "automatic OC" and placed hard limits on voltage: the previous-gen Maxwell/Kepler/Fermi cards could be overclocked a lot more than the current Pascal ones. So I suppose if you keep the temps under control it's not likely that you're gonna damage your Pascal cards, nvidia just won't let you.