Yes, I believe that flasher doesn't like the ubinized file when it's compressed with zlib. However nandwrite isn't as fussy and just happily, well, writes the data to the mtd5 block - the reason that flash_eraseall is needed is so that the ubi controller doesn't get in a tizzy and fail to mount the device once attached.

I'll test later (to simulated NAND) if it's possible to use nandwrite with the file created via mkfs.ubifs, i.e. prior to ubinizing, if ubiupdatevol is called first. It probably won't work, but worth a try anyway.