I've used and use both and have never damaged a card. How do they cause damage, over overclocking? You really should explain that more.
I'm no expert on this, so that's why I use caution when I use either. Mainly it is overclocking and stress testing that can fry them. Also the fan settings can stop the fans spinning which will also fry them. Apparently soft modding in Riva to unlock pipes can also damage the cards. This is a warning that comes up when you select the "Power User" tab in Riva, and I don't think they are joking.