My personal datum on this issue: I moved from Berkeley California and when I visited 4 years later I had constant ringing in my ears until the visit was over. I'm assuming it was related to the installation of smart meters on all buildings, which had happened in the intervening years. I was sleeping in a house with the meter right outside my window. The walls were thin and included little cement.
I also experienced that using a cheap "feature" phone in India for conversations more than 30 minutes caused my hand to heat up and actually ache. Probably no shielding whatsoever, no idea what the mechanism of injury was.
I don't believe it's impossible to set up a study of these issues. I really hope 5G will be withheld until they have a clue what it's effect will be.