DC DC converter control design, PID vs compensators
A type III compensator and PID controller are essentially similar.  A voltage mode compensator in a SMPS usually puts a pole at the origin (integrator), and a pair of zeros somewhere around the LC resonant frequency (differentiator=zero) and provides a way to adjust the gain (proportional term). (The final poles go maybe at the ESR zero and half the switching frequency to reduce noise.) A PID controller provides a pair of zeros and a pole at the origin, so if your PID implementation modifies the open loop gain so that it's stable with good gain and phase margin you have accomplished the same thing.

Some SMPS controllers have a tranconductance compensation amp so you are limited to putting compensation components from the output to ground, but if you're putting together your own control loop there's no reason you can't use your own implementation of PID control.