Across most kinds of games, there generally exists some kind of feature that is presumably designed to help ease new players into a game until they build up a greater familiarity with more intricate and advanced gameplay mechanics. At the most basic and simplistic form, this may be a tutorial or introductory level that teaches players the controls. A more subtle example may be the Assault Rifle from the Halo series—simple to use, no particularly noteworthy learning curve, and utterly outclassed by other weapons as player skill increases, but it's not entirely useless given the right niche circumstances. In Overwatch, this takes the form of quite a number of Heroes who, once again, aren't entirely difficult to understand but once you get to the higher Competitive brackets, they start to lose effectiveness.
The most important part of the aforementioned maxim however is that such "introductory" Heroes have to lose at least some amount of effectiveness as player skill increases. After all, if fire and forget Heroes like Symmetra or Torbjorn were as effective in the Overwatch League as they presumably are in the Bronze Competitive bracket, then what motivation would one have to improve? Not only would it feel bad to be outperformed by a Hero who requires such relatively minimal player input to the point that you may as well be fighting the game and not the player, but it would be rather embarrassing for the image of the game itself if people found out that the habits that we commonly associate with skill contributed nothing to winning. Yet, through a rather absurd series of events, this is more or less what happened with Mercy.
Given that Mercy is arguably the most prolific healer in Overwatch, little needs to be said about how her Ultimate ability rework proved to be a retrospectively poor idea. It certainly didn't help matters much that Mercy was in literally every single Overwatch League match, oftentimes undoing amazing plays with a rather disproportionate amount of effort. That's not to say that Mercy, being (arguably) the best single-target healer in Overwatch, doesn't have a place in the game, but red flags should've definitely been raised if the best players in the world practically unanimously decided that the easiest healer to play in Overwatch was also the best healer in the game. Mercy's pickrates in normal play echoed what was happening in professional-level play, with Overbuff claiming that she had a staggering 14% pickrate across all platforms in Competitive mode. Once Blizzard decided to take action and nerf her, Mercy's pickrate settled to a slightly more reasonable 8% pickrate, but it showed the dangers of making easy to play Heroes excessively powerful.
Funnily enough, the inverse can also be true- a potentially powerful Hero can be so challenging to play that, even if you were to master every aspect of that Hero, practical problems will prevent said Hero from being used to their full potential. Unfortunately, a number of Heroes fall into this hard to learn, hard to master, yet narrow in usefulness category. Sombra for instance offers a great mix of damage and utility, but unless you're playing with a well-coordinated team, you're probably better off as Tracer. Ana has amazing healing capabilities and quite a useful set of debuffs, but the prevalence of barriers and the power of mobility in Overwatch makes her a very questionable pick at times, especially now that Moira exists. Blizzard has apparently realized that a middle ground must be struck with their proposed Sombra tweaks on the PTR by shifting some of her power away from the utility aspect and making her more of an assassin than a pseudo-support DPS (unfortunately enough for Zenyatta players), but there's a long way to go before some of the more problematic Heroes of Overwatch can reach the sweet spot of being easy to learn and hard to master, yet fair to play against (and fun to play) and viable in a reasonable number of situations with a few obvious and easily exploitable weaknesses.
Perhaps, if we are to look at Mercy again, the answer lies in obliterating the notion that there has to be some kind effectiveness cap for certain Heroes. After all, in the hands of someone like JJonak in the Overwatch League, Zenyatta may as well be a sniper but for Heroes like Mercy, there's only so much that you can do regardless of personal skill. Frankly, so much of Mercy's power comes from her team that the team is actually punished as a whole if you don't have the least skilled player play as Mercy, or at least that's the impression that comes from her abilities, creating a vicious loop where there's not much to learn while playing as her. You can't give Mercy the same potential for damage for obvious reasons, but by making it so that her own survivability is no longer entirely dependent on her team, then maybe some of the core problems surrounding such a necessary Hero will be less of an issue. Modifying Guardian Angel so that it actually functions as an escape ability rather than a "please help me teammate" ability (i.e. giving Guardian Angel some kind of reduced speed and limited distance flight capability when a teammate is not targeted for instances where you can't or don't want to fly to a teammate), tweaking Mercy's sidearm with a slightly faster projectile speed and or fire rate so that it's a respectable weapon rather than being a thing you use to disrespect someone, and a very small increase to Resurrect's range could contribute a lot to making Mercy more enjoyable to play without changing her identity as an extremely vulnerable healer.
With a 27th Hero on the way, it would be impossible to make every single Overwatch Hero viable in every single situation, but at the very least Blizzard can make it so that some Heroes can offer more to the team via abilities that promote a certain skill whether it be accuracy, positioning, or at the most basic level, teamwork. Fortunately, Blizzard appears to be doing a balancing sweep across the board, but given how long it took for Mercy to reach the somewhat unsatisfactory point where she is now, it may be best not to get one's hopes up. On the other hand, the Overwatch League and Mercy's reign over the game appears to have been a wakeup call for Blizzard to make sure that half the roster isn't ignored, so while things may be moving slowly, at least they're moving at a steady and presumably measured pace.