I've dealt with XFX and ASUS support in the past, but neither one recently. I never had a problem with either other than when XFX's support website malfunctioned a lot about a year or two ago and wouldn't allow any account logins or new registrations. My last ASUS-related experience was more than 5-years ago, so take that with a grain of salt. I've never dealt with Sapphire.
One thing XFX always did for me was answer the phone when you call the toll-free tech support number and attempt to assist me with whatever issue I had. My brother had to RMA two different XFX GPUs over the last 8 years, and each time he received replacements roughly 15 days later. But the last time that happened was nearly 4 years ago...
Sadly, XFX has gone the way of the other manufacturers and dumped its lifetime warranty.
http://xfxforce.com/en-us/support/xfx-warranty
I quote the policy:
The following Graphics Cards are eligible for an extension of the standard two-year limited hardware warranty:
XFX Radeon R 200-Series Double Dissipation Edition and HD 7000 Series Double Dissipation Edition Graphics Cards.
XFX Radeon R-Series and HD 7000 Series Graphics Cards with 10-digit model numbers ending in “R” (example: “FX-797A-TDFR”)
XFX Radeon HD 6000, HD 5000, and HD 4000 Series Graphics Cards
XFX GeForce GT 640, GT 630, 520, GT 430, 200, 9000, 8000, 7000, 6000 Series Graphics Cards purchased after April 17, 2007.
If you register any of the specified products noted above online at http://www.xfxforce.com/ within 30 days of purchase, your limited warranty will be EXTENDED for the duration of your life. Registration within 30 days of the date of purchase is a condition precedent to receiving the lifetime warranty.
Note that the R9 300-series is not listed among the cards eligible for the lifetime extension, so I highly doubt it would apply to the R9 Fury cards.
At this point, it would appear that ASUS and MSI now offer the longest warranties (in the AMD camp) at three years, each. The others now only offer two-year warranties.
For all I know, XFX may have also dumped its support for aftermarket cooling solutions, making (last I checked) MSI the only manufacturer to officially support it. (Provided that should an RMA be necessary, the card is returned to original condition prior to shipping.)