Not necessarily more than any other card. As long as it's been running within component specs (especially temperatures), I wouldn't be worried.

It's obviously been running fine under high loads, so why would it just decide to stop working fine?

I think the major issue is that many cards have been running outside of specs for a long time. High loads tends to increase the risk of that, so the problem lies in figuring out the case for the card you're interested in.