Won't work for metastasized cancers for obvious reasons. Perhaps it will also be limited by bones just like regular ultrasound where their echoes produce shadows. I could be wrong though.
Ultrasound in general is also operator dependent: the skill of the human doctor matters. Limits scalability.
In rodent studies, it actually does help with metastasis. It breaks tumor cells apart in a way that makes them recognizable by the immune system, which goes on to kill other cancer cells throughout the body. We'll have to see what happens long-term in humans treated with it.
Ultrasound in general is also operator dependent: the skill of the human doctor matters. Limits scalability.