Maybe I've been unlucky with my cards, but I have started mining last year in July, and I have 4 dead rx 470s on my hands. I got my money back from RMA process so it's not so bad, but it seems like my failure rate has been higher than anyone else. Prior to mining I have never RMA'd a single piece of hardware... And the cards were kept in good condition or so I thought, always below 70c in an open air bench.
Ive had a similar run. Of the AMD cards ive so far bought, ive had a failure rate of about 10% over the first year. Of the Nvidia cards, they haven't failed during usage, but about 5% were DOA. All of these were under warranty so RMA'ed but a pain nonetheless.