Except that she can be killed before she rezzes them all and 10 seconds of 5v6, for example, is quite long enough to snowball the fight.
The impact of her rez has been significantly lowered since getting two picks will still give you a numbers advantage and again, no immunity while rezzing means you can kill her as soon as she does and prevent any more after that.
Note that they'll also be nerfing her damage so that infinite ammo battle Mercy isn't a better dps than the dps themselves.
Edit 1: Good Mercy player will probably take full advantage of her rez CD resetting when she pops her ult giving her up to 4 total rezzes in the 20 second ult duration. She can be pretty hard to kill while flying around and her passive regeneration does not get interrupted by damage so her being able to live long enough to get those for rezzes is very possible. I think they might end up nerfing the ult duration and/or ult generation else she might just be able to rez almost all of her team, albeit one by one.
As I mentioned above the positive spin here is that other than the back to back rezzes her ability to stop the snowball caused by picks is severely reduced and if focus fire is properly directed it could effectively be nullified.
Edit 2: Mercy and D.Va apparently don't care about your CC abilities. some bugs that will need to be foxed for sure.
YouTube vid:
https://www.reddit.com/r/Competitiveoverwatch/comments/6vvsji/mercy_and_dva_arent_reacting_to_stuns_correctly/**