Originally Posted by Fogojam
Both of the abilities you mentioned dont take effect until several seconds after they've been cast. So in other words, Twitch had likely already begun to go invisible before he was stunned, and likewise Janna had already started charging her tornado before she was stunned.
Just saying, Janna's whirlwind can be set off prematurely by pressing Q again, although it was less strength and doesn't travel as far (based on how long it builds up for)
But yeah, stunning imo wouldn't be disabling completely, if someone is stunned, they find it harder to do things, for example their accuracy would be off and they'd move more slowly... just my opinion though.