I haven't tried to boot it, but as I say, I believe flash_eraseall to wipe the mtd5 partition, and then nandwrite of the image over the now-clean partition will work better than flasher. It definitely reads it properly when I use nandsim and mount it (using mount -t ubi1:rootfs /tmp/mnt/rootfs) so I see no reason why it wouldn't - after all, it's only a partition at the end of the day. There are no rogue dmesg errors either (there are loads though if you use ubirmvol and then ubimkvol rather than flash_eraseall, and the same if you use just flash_erase to remove the first block - I've tested just about everything, except for ubiformat, which complains about lack of memory and just falls over).