The failure in the provision of evidence, is with those that claim it is safe. Its not for me to prove it is harmful.
Ask yourself, how can a treatment that breaks kidney stones or burn tumours also be 'safe'? It may be that the impact is negligible, but that is not safe. In the womb especially, a slight impact may have a large effect on a growing foetus.
Ask yourself too, what level of radiation is safe? How much microwaving? Is any?
So am I fearmongering, or is this a valid concern? I think it is clearly a valid concern. I do not simply accept medical consensus over the reasoning of my own mind.
I can see use cases where the information that is provided by ultrasound is useful and outweighs the risk. I think that casual, repeated usage of ultrasound for pregnant women, for the 'gender reveal'/colour scans or whatever, with powerful machines, is ill advised though. I don't recommend anyone undertake any unnecessary medical procedures.
I also think that an adult that has awareness of the scope of a potential impact, can do what they want - its their choice to do as they want. I object to inflicting avoidable harm (albeit the harm might not be obvious) to individuals that cannot consent - eg infants, foetuses.
FWIW, I do have kids, and - to my regret - I did scan them before they were born. I wouldn't do it now, knowing what I do.