Stun affects for howitzers that fire multiple shells would be too much, other affects like "shell shock" that would lower the DPS could be allot more balanced.
I agree on stun. I don't like mechanics that take away control of unit. Suppression would be better.
Suppression for multi-shell howitzers is a way to make them do at least something against infantry (holding them in area of barrage for some time through speed reduction). You can argue that they are not supposed to be used against mobile units, but in combination of reduction of lethal radius that I've mentioned earlier, I think it would be fine.