Stun is just like all other status-problems and their durations react on the same times of each other. Stun is also very powerful when in certain situations, especially on certain bosses, like the Jelly King. After each tantrum the King throws, you can easily use Stun and lengthen the phase after his tantrum. Sleep I believe is far worse than most vials, almost next to freeze or stun, which both are also useful in their own parts. While Sleep immobilizes a foe AND stop them from attacking WHILE healing, they stop after a while, or don't sleep at all. Plus, against hordes you just want to kill off, Sleep and Freeze are useless, like on the Jelly King. Stun is supposed to only make the foe slower, attacks and walking. Stun and Freeze are very effective on their own, Freeze for stopping completely and Stun for slowing down. Sleep really needs a buff, but Stun, not really. And times change between "Minor," "Moderate," and "Strong," which last for different amounts of time on their own.

Stun is viable. The biggest problem I've experienced is that stun creates a delay between an enemy's "starting to attack" animation and the enemy's actual attack. This creates two problems:

1) Stunned enemies will seem to attack without warning. They did give a warning 8 seconds ago.
2) When coupled with any major latency, some enemies will attack faster than the animation plays out. The result is you will get hit even though you are nowhere near the enemy.