"flashing" a phone is largely the same as any OTA update. There's of course always a risk of it going wrong, disk failures are always possible, but it's exceptionally hard to do so accidentally. Especially with custom ROMs where they basically never include a new bootloader, so "flashing" is no different than installing an OS on a desktop system - it's just writing to the boot partition. Which you can always do again since the bootloader is still available.