I've only ever had, I think, two mobo's fail. One was dead, right out of the box, brand new. The other was, I believe, the result of a significant mains fluctuation. Quite what/how? Dunno. Surge from nearby lightning strike? It was a long time ago.
The more I think about it, the less sure I am that that nailed the mobo. It certainly nailed both a £1200 (in those days) graphics board, an Adaptec 4-port hardware RAID board and all four hard drives. And it was a big enough jolt that it physically blew large chunks out of some CMOS on the hard drive controllers, and my office stank of that acrid electrical burning smell for weeks. Literally, weeks.
What surprised me was that the rest of that PC somehow survived it, and the RAM certainly did. The more I think, the more I wonder if the mobo did, too.
Of course, naturally, this occurred about 3 weeks after my UPS battery gave out and while I was awaiting the replacement's arrival. Grrr.