so is it the undervolting that is killing the cards?
I remember I ran my 290x (single card, just as a hobby) almost 24/7 at 80 C for months and month with no issues. But 100% STOCK.
Damage from undervolting is not normal. It's safer on the hardware but it loses stability when pushing it which can cause the driver to get buggy until a reboot. Overvolting is more damaging on the hardware end.