That's basically just the same thing as Zarya and Pharah using their ults, isn't it?
Not exactly.
There are many ways you can mitigate those situations. Have a Zenyatta? If he ults, you can basically keep everyone alive, since the damage is done over time and not unloaded in a huge burst his heal would overtake it. You can also shoot her out of the sky before she finishes the ult. Reinhardt can also block a bit of the damage allowing someone to shoot the Pharah before she unloads all her damage. McCree? Same deal if you block or headshot him before the cast. Hanzo? Zenyatta's ult can power through it's damage. Etc, Etc. Lucio Ult, etc. So on and so forth.
Doomfist is COMPLETELY UNATTACKABLE while in Meteor Strike mode. He leaps up so high no one can even see him, allowing him to line up his attack. The attack also unloads all damage at once at it's epicenter, and IGNORES BARRIERS, meaning if a Zarya and Doomfist queue up, you are dead. A Zenyatta won't save you (since all the damage is frontloaded, the Zenyatta would only save himself), a Reinhardt can't mitigate, and you sure ain't killing the Doomfist before he comes down on your face since he is unattackable. The only one that might succeed to prevent death to everyone in the ult radius is a Lucio ult.
That is the thing, Doomfist's downside to his Ult is that it has a decently long "activate" time once you line up, which also shows the enemy the impact point, giving them time to run and escape or at least move to the outer edge, where the damage is less deadly. If they are held by Zarya though, you won't be escaping that epicenter damage point and there is nothing you can do to stop the Doomfist.
P.S. I also forgot to mention that to often get the most out of those Pharah / Zarya wombo combos, the Pharah needs to position herself in a way that takes the other team by surprise, but this can leave her vulnerable if she is caught trying to flank. A Doomfist using meteor strike has, basically, invulnerable stealth. He can cast it outside the room the attack is happening, fly into the room flying over the heads of all the tanks, and then land right on the healers, with no risk to him until after he finally lands. I saw a pro player even use this to escape death once, by meteor striking then flying all the way back to his team after getting caught in a situation he couldn't win.