EDIT to add : Flesh mobs also have a couple of states that make them insanely resistant or totally immune to stun. such as their charge
"A stunned enemy will very quickly return to being a threat, a dead enemy never a threat again... unless its corpse has janked out." - Johnny Helldiver
being honest, it SUCKS that stun is treated as such a forgotten element of the game. especially with the recent changes to the other CC effects (fires panic and gas's slow).
i don't think any enemies in the worth stunning but immune to stun should be instantly stunned by the stun grenade, but if you stick an urchin to them they should slow down with every tick until their completely imboiled after the 3rd or 4th.
it would also be cool to see on the larger multi part enemies like factory striders and vox machines. being able to stun their minigun turrets or other weapons in the build up to fully stunning them.
I don't think stun should be buffed to a "meta" pick, as we arent here to shoot clay targets but i think theres a healthier middle ground between what we have in game currently and that undesirable potential.