the perfect balance in a game like Overwatch is an unattainable illusion as perfection is subjective. There will always be stronger and weaker heroes. But it is possible to raise all heroes to a usable level and to slow down too strong heroes.
But Blizzard does a very good job of failing to do this. Blizzard lacks a philosophy, a guiding principle for balance or is constantly changing. Half a year ago, before the + -25 HP patch, Overwatch was in a pretty good spot, but this was undone with the following patches. This shows the absence or the constant change of a guiding philosophy.
Such a philosophy is likely to come with OW PvE. However, it is also made very easy by deleting a complete player per team and thus removing a lot of complexity from the game, which makes balancing much easier. But this also shows the inability of Blizzard when they have to make a game simpler in order to be able to balance it and follow a philosophy.