On my 5 years old cubieboard server I experienced NAND corruption more often then not when power outage occurs. Never had this kind of problems since I switched to SD cards.
In my experience SD cards are much slower but more reliable and easier to manage (backups and such).
There's no real inherent reliability difference between SD and eMMC, it's all down to the individual controller implementation. Though some SD slots are quite flaky, i.e. the original RasPi models.
The eMMC that orangePi uses seems to be more reliable than the average SD card. Of course there are good SD cards out there and bad eMMC.
