Yocto is a bit too much and uBoot is a bit too little.
I generally use Ultibo, it's about right.
Single purpose applications that just boot and run fast.
Buildroot is more flexible but the options are so many.
Barebox is smaller, much more restricted.
Add to Barebox and or take away from Buildroot?
Both I can see as useful for embedded Pi's
Just more software tools in the Pi tool box.
If I can master Barebox then a multiboot GRUB/Lilo type option might be doable.
Could be usable as a restore/backup system too.