Nerf the Pharah/Mercy flying a little. It is not unbeatable but it is overpowered compared to other combinations of characters.
For starters, for many hero's it is extremely difficult to kill Pharah when in mid-flight anyway (Reaper, Symmetra, D.Va, Reinhardt or Junkrat for example), but to combine her with Mercy flying overhead and anything short of a Bastian won't hit them accurate or hard enough to kill them.
The trick is to take out the Mercy first before Pharah because if you try to kill Pharah first, any damage you are occasionally accurate enough to hit won't be enough to compensate with Mercy healing her. Aside from Bastian, Soldier 76 with an accurate player is pretty effective against Pharah on her own, but with Mercy healing the need for accurate and constant hits is a much greater requirement.
I am not saying ban this flying/healing combo, its clearly a skill to have two team-players working together in such an effective way but it is overbearing on virtually any team combination without a Bastian, and maybe it can be counteracted by adding in a "glide" timer so that she can only jump/glide for 2 seconds before dropping down, so as to give a small window of opportunity to the attackers to attack Pharah before Mercy reaches her and heals her again.

I just feel something needs done because I have played a lot of games where teams are fairly balanced and all is going well, and then suddenly one of the enemy turns to Pharah, and has Mercy suddenly hook up into a combo and thats it, game over, Pharah is getting 15 player multi-kills easily and all 6 of your team are all focussed on trying to take down an almost invincible Pharah that is kicking their ass while the other 4 heroes on the enemy team are capturing points or escorting payloads and you can do nothing to counter it as your bombarded by semi-invincible Pharah and her "Rocket Attack" ultimate every 60 seconds.

Blizzard is readying a patch to address the issue that the game is still running in Switch 1 build and just 30 fps.