I have a couple of M2 packs. They are well built. I keep one attached to the outside of a Condor 2 backpack and sometimes wear the second on my belt.
The M2 sticks out further when worn on the waist than my other, smaller holsters (e.g., a multitool or flashlight holster). Unless I am paying attention, I sometimes hit the M2 on the arm of a chair when I sit down. It is comfortable when driving -- no problem there.
The only other time I can see an M2 being uncomfortable when worn on a belt is if you load it up with too many heavy items or if you wear a long, tight coat.