BackupMenu is good. In fact, in many cases, stuff invoked from /sbin/preinit(for example, a shell, or BackupMenu), is sufficient to fix messed up things. Under normal circumstances, you won't need a rescue os. But:

-BackupMenu has to be already installed before you break your things. It can not help you afterwards, if you didn't have backups(you should :-) ).

-It can not help you(say by providing a shell) in some worst case scenarios (say a borked busybox, broken libc, syntax errors in /sbin/preinit prior to backupmenu.sh execution), simply because it is not designed for that and couldn't do it by design. Flashing would be required anyway.

So, RescueOS and BackupMenu are two different things which can hardly be compared. RescueOS has been created for one simple reason: There wasn't imho a non-PITA solution known to me out there to mount your rootfs and access the EMMC partitions with a broken maemo system.

Once RescueOS can charge the battery, I think more people will find a usecase for it.