There are many studies showing how harmful they are, and there are many showing they are not. A lot of studies were funded by tech companies. As someone who understand incentives, I tend to err on the side of caution.

And out of basic logic and knowledge of cell biology, never in the history of mankind we were exposed to such amounts of electromagnetic radiation so close to our cells (which essentially work by electric interactions). Radiation intensity is inversely proportional to the square of the distance.