At least I have some device where mtd4 is totally locked due memory errors and which are always inialized as readonly. Thats why it might be nice to have initramfs or mmcblk based version of ubiboot... Some people ofcourse are lucky and have working still mtd4
I have been trying to look up any references to problems booting extended partitions, or possible special naming needed for it but with no success really.
A easy case to solve if you only had serial console on your device