This is always tough to gauge, IMO.
Reliability of the motherboard itself is a difficult topic due to the many variables that come into play. I'll present what I feel are the most important, with a couple quick questions for each, then my personal experience.
1. The user. As with anything, when customers get to touch something, things can go downhill quickly.
-Is this an experience user or new?
-Did the user adhere to ESD standards?
-Does the user RTFM, or just tear into it?
2. Power
-PSU, is it quality or cheap?
-How's the building power? Is it known to have problems or stable?
-is there a UPS involved and what's the quality for that as well?
Ok, my personal experience...I'm not counting the different chipsets are part of my XP, but have used VIA, Intel, nVidia.
I've personally used these brands to my recollection: Gigabyte, Asus, Epox (gone now), MSI.
Gigabyte - Always been solid. Never had to RMA
Asus - almost always solid. I RMA'd a board for the first time this past fall. The onboard NIC was physically not working. The logic worked, could install drivers, but the NIC never detected a cable. This was not the OS, as I tried a couple. Their RMA process wasn't difficult, but their support reps did have me do some stupid shit before the RMA. Like clean the NIC port....lol ok guys...
Epox - RIP - never had a problem with the couple boards I owned.
MSI - GFY - had weird issues with a couple of their products, looking back on it more than likely just firmware (BIOS) issues...not show stoppers....side note: they burned me on a $10 rebate 10 years ago....so they can GFT....not the only player in town.
I cannot comment on any other brands.