The issue I have with PWM is that you can't do extended hop stands at specific temps like you can with a PID.
With a PID the boil vigorousness is controlled by setting the duty cycle from 0-100% by using up/down buttons to change the number. With a PWM device you simply turn the knob so it's faster. Both control the vigorousness of the boil well, but PWM is faster to initially set. The thing is, it's not something you play with. You set it once and leave it, and leave it the same way forever.
For example, I use a PID and while getting up to boil I run the PID at 100% to heat as fast as possible. Once I get to boil I press the down button a few times to get to 85% and then leave it there for the whole hour or two of boil. There's no reason to play with it.
If I want to then do a 80-90 min extended hop stand after the boil (ex: Heady Topper), I switch the PID over to auto mode and enter the temperature I want to hold. A PWM can't do this. It's manual only.
A PWM device could maybe shave a second or two off the time to turn down the power while boiling, but then you can't do 80-90 min hop stands held at specific temps.
I looked at PWM devices when designing my panel way back in 2008 but decided against it to not limit what I could do in the future.
Give & take. YMMV!
Kal