I personally prefer to approach the topic of "safety" by considering the trade-offs. The knowledge gained through ultrasound significantly outweighs potential risks associated with it.

People still continue to play the lotto thinking they will win, and they reject statistically low risks in lieu of a greater risk created by avoidance. See: any vaccination topic.

When shifting into the topic of a wearable though, the extreme amount of time alone amplifies the risks into outright dangerous levels. I did not seriously believe ultrasound to be safe to that level.