Many Many Variables
Safe is a relative term & always in the eyes of the beholder. Microwave is just electromagnetic radiation (below infra-red, above radio in frequency) & a method of heating water (as in a microwave oven).
If cells get hot enough, proteins are denatured (as in cooking), no longer function, cells damaged/die. So this is just a way of targeting tissue. Key: how applied, controlled & collateral damage.