Kill the mercy first then kill pharah. Hitscan should be one of your dps picks anyways for a rounded team comp. In League Pharah Mercy would be a fairly standard Carry with Soraka or Janna (dedicated healer/support). It’s a difficult combo to deal with but not overpowered, usually focusing the healer first is a good way to deal with the combo as their sustain has been removed. The carry lacks self sustain (usually) and disengage so killing their support that covers both is a good option.
They can’t handle the slightest amount of cc or team focus. They are indeed annoying but any amount of coordination for the team will kill them pretty quickly. Their damage is pretty low on armor based heroes unless they get close or they use their ultimate.
I don’t play either so I can’t give too much input on what parts are needed to nerf them without being too damaging to the kit. I may not like Genji or Tracer, but I will not advocate for something to be nerfed to unplayablility. Maybe put a cool down on double jumping (like Hanzo), or not a full heal for Tracer’s recall. Tracer and Genji are a lot like Ekko and Yasuo (League) they are hyper carry mobile heroes that have many options of entry. Both are hard to deal with even if they are average or behind so playing characters that can resist their damage is a good idea.
The reason I make the comparison to League is because I have dealt with similar situations in League to Overwatch, the only difference I see is the format in which the situation occurs. For League, though you can’t change your character, you can still purchase items that would be better suited for dealing with problem characters such as building magic resistance for Ekko who deals magic damage or building health and armor to deal with Yasuo. This itemization can be paralleled to picking different heroes in the same role that you are in (ie picking Reinhart to counter Brigette when you were previously Winston or switching from Junkrat to Hanzo or Widowmaker to deal with Pharah-Mercy).