For those having ubi errors: the error seems identical to the one here with wndr4500v3 (almost identical hardware, too): WNDR4500v3 - Kernel Panic - not syncing: VFS
I was having exact issue too on my wndr4300v2 (first boot after flashing openwrt factory img was fine, then rebooted to bootloop), and sysupgrade solved the problem.
Btw uboot does not work at all with my uart adapter. Switched to breed for wndr4500v3 and it works perfectly.