Like anything else, PWM can be done very well or it can be done badly. Like Dissonance said, there are some mods that do it very well like the Provari which is arguably the most accurate regulated vaping device on the market. And then there are others that don't do it so well. A lot has to do with the frequency of the pulses. For example the Provari 2.5 uses a pulse width frequency of 800 Hz, or 800 time per second which is fast enough to provide extremely smooth regulation. Many cheaper devices use 33hz, or 33 times per second which is noticeable while vaping and has a common label of rattlesnake vape. So nothing wrong with PWM, it just has to be done well.