I've been running my 8 7950's at 100% for 12 months now and not a hiccup to be had. But as with any consumer product ever made, each individual card, even in the same series, is slightly different with varying weaknesses and strengths. Perhaps I've been lucky. It's incredibly simple to just buy a new fan if one were to burn out. You can even take the chance to upgrade to a more powerful fan if you want.
Which brand are your 7950 cards?
Sapphire 7950. The 100352-4L model specifically. I've had them overclocked since December with my core at 1050, my memory at 1700,and my voltage at 1068. Out of 8 cards, only two dont like an overclock and I even run them at a slightly higher 1075 voltage. Otherwise those two cards will crash my drivers for whatever reason. It could be my drivers, but I haven't really cared about getting an extra 200 khs for the effort of troubleshooting past what I've already done.