Alrighty, so I've played a few matches against a few DFs now, both good ones and bad ones. (One particularly good enemy DF completely decimated our team single-handedly and ended the match with the 'MVP' vote card for being on fire for 70%+ of the match. I was on Pharah, and although I never died to him myself, I was unable to kill him due to him being pocket-healed the entire match and him continually getting extra shield health from beating up my teammates. His crazy movement also takes a bit of getting used to, when you're trying to lead your projectiles. This particular curbstomp was almost certainly due to bad matchmaking, and/or someone smurfing. But I digress...)
The ult is still bullshit, uncounterable, and almost always a free kill on any low-mobility target. But I notice teams are now starting to scatter whenever they hear it go off (in the same way a D.Va bomb or Junkrat tire causes teams to scatter apart). Because even if DF gets a free kill out of it, if you're all split up he's only going to get ONE kill out of it and not any more than that.
I also discovered that there are a few 'vulnerability frames' at the end of DF's ult; there's about half a second after he lands on the ground before he can move or use any abilities, THAT'S the time that any meteor-strike survivors on your team need to all shoot at him. He goes down very quickly then, especially if he's ulted right into the middle of your team.