This of course is biased but here we go.

When I am tanking I get 1/2/3 commendations often times 2 if I pulled quickly I get 3. If wipes were done I apologize and excuse my bad playing because reasons (changed keybindings, pulled too much).

As a healer no matter what I do I typically get either one commendation or nothing at all. This happens even if I put a lot of effort (dps any time healing wasn't needed, nobody died.

As a dps I don't have much experience, but pre-expansion I would get 1 or nothing at all.

A lot of people leave a run after its done never bothering commending.