I think that Assembly would be part Rock. It seems pretty obvious considering it's design. Not sure about the other ultra beast.
Rock definitely looks obvious since it's meant to be this giant wall of bricks. The lit-up lines seem to suggest Psychic for a secondary type. Unfortunately, Rock/Psychic is one of the worst defensive typings due to having seven weaknesses, though unlike the other type combinations with seven weaknesses (Grass/Psychic, Grass/Dark, Grass/Ice, Rock/Ice, Rock/Dark) it doesn't have a 4x weakness, so that will be a plus.
Burst's type looks more ambiguous, but I'm guessing it'll at least be Fairy. Maybe Ghost or Psychic for a secondary type. I figured I'd pick Psychic for these two since S/M's version-exclusive UBs shared a type (Bug and Fighting for Buzzwole/Pheromosa, and Steel for Kartana/Celesteela).
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