This is all dependent on how the missile barrage works, what the range is, how much damage it does, what it's CD is, etc.
This can either shuffle D.Va along into uselessness or make her an utter monster.
I tend to use DM in small bursts anyway unless I'm just in the middle of a big team fight, so that itself isn't bad for me, except for the fact that I will no longer be able to shut down any long lasting ults, like Soldier's Tactical Visor. That part makes me a sad bunny.
This is the update I'm most excited to see. I really wish I had PC overwatch so I could try it out on PTR when it comes out. But as of now, there's nothing to feel because we don't know anything about this new ability.