[34 / 2 / ?]
What if the reason we saw a signature move for UB Burst but not UB Assembly is because UB Assembly's move involves Unown and they didn't want to spoil the UB/Unown lore hinted at here:
https://archive.nyafuu.org/vp/thread/33064413/#33064506
https://archive.nyafuu.org/vp/thread/33064413/#33064506