My recent experiences:
I don't think XFX does a lifetime anymore, my HD 7850 isn't. However my HD 7850 has been pretty reliable for the last year and a half. I have two XFX HD 6850s that were run non stop by a friend of mine before I acquired them and they are solid too. XFX was pretty friendly to me when I asked them if I could remove the cooler and replace the thermal paste, they said okay and said it wouldn't void warranty.
Gigabyte RMA was pretty easy to get approval but it took about 3 months to get a working refurb, first time they just sent the same card back and said it tested okay. I sent it back, finally got the replacement 3 months from the start of the ordeal, and I sold the refurb. The refurb was obviously a repaired card. At the time, they were going through a supply shortage due to litecoin miners frying 7970s and RMAing them so I likely received a repaired card that was in the door out the door.
Haven't RMA'd anything else recently. With everything you should register it immediately, retain all copies of receipts, and try to make sure that you do not physically damage any component, don't damage the circuit boards, or mess up the tape on the screws for the VGA unless you get approval to remove the cooler.