User blog comment:JmLyan/Arila, the Gun Mage/@comment-25996958-20130712162025/@comment-7481207-20130712215154

It's veyr unreliable though, as hitting champions with it constantly reseting the shot count means you have to shoot minions with it to build it up. But, if you happen to get a marked shot against a minion while building the probability up, the counter is reset and you have to start over. And while the cooldown is low, the statistical probability of getting it off if you only shoot champions is 1 stun per 12 shots, the cooldown could be considered 36 seconds...

And veigar's stun is AoE and more reliable that stuns for up to 2.5 seconds on a 16 second cooldown, so I'd much rather take his stun even with the longer cooldown (although technically, the stun itself on her Q has a longer cooldown.)