I think his warning is appropriate, not so much because medical ultrasounds are dangerous, but because your homemade device probably doesn't have the same level

I think his warning is appropriate, not so much because medical ultrasounds are dangerous, but because your homemade device probably doesn't have the same level of testing and oversight that your doctor's does. We know that ultrasound machines can cause warming and cavitation, and while these effects are minor when part of an ultrasound at the doctor's office, it's not hard to see how a homemade device that emits more radiation than necessary could exacerbate those effects.