For a mod with a variable output, all of them use PWM so you can't really say that PWM is bad. It depends how it's used. Power converters use PWM to drive voltage higher or lower, but they do it at a very high frequency and they filter output to purely DC. What you get at the atomizer is the same kind of voltage you get from a battery.
Some mods use a PWM driver to provide a non-DC output at the atomizer. This is probably what you're talking about. In that case, you get a pulsed output, it's not like a battery. The amount of power delivered to the atomizer depends on the voltage and width of the pulse.
Power is power so on paper there should be no difference in delivery whether you're using a purely DC voltage or a pulsed voltage. However, people claim there is a difference. I can't say myself since I've not used a mod that provides a PWM output. All of the ones I make myself use a power converter that provide a flat DC output. There is the advantage of accuracy. A mod that provides a PWM output at the atomizer is not going to provide readings as accurate since there's a conversion in scale that has to be done.
