As you can see in the video, heavy attacking and then casting Mist will cancel the Mist, forcefully pulling you out + wasting magicka. This is a major problem, especially for kiting in PvP where sustaining stam within Mist is crucially important.

https://www.youtube.com/watch?v=_9HoGdeyIec

There are also several other bugs with Mist form not shown, such as being randomly pulled out without any interaction from the caster, and also being unable to cast it whatsoever (although this may be an issue related to the current skill delay and lag in Cyrodiil).