As far as PC goes, it is much easier and reliable to play Mercy by using the pistol, not only with On Fire and POTG potential, by creating windows of opportunity to do your job whether you want to or not.
For example, there are a lot of occassions where I no longer try and heal and damage boost a player anymore because it is far more rewarding and in the end, safer for me to use my pistol. On PC where aim is much more accurate at all levels of play, I am better off helping kill the enemy to either heal the person at the end or make a safe revive because the power level Mercy holds with her support features (healing and boosting) often are fairly weak in a lot of scenarios, including 2v1’s where the support factors can still perform to a weak enough level that it is better to kill the enemy yourself and just revive or heal later. This is true with helping your team be more aggressive as well and trying to be that more aggressive support that you could be previously, whilst this does not apply to making risky heal plays that could save a life by giving a squishy a higher chance of being able to peel from a fight, instead you distract and attack and make room to heal your team. Again, this is regardless of whether you want to or not.
Peeling to me is the primary issue that we need to cover.
Peeling is a necessary aspect of this game, having to peel for too long to get healing goes against the fast paced nature of the game.
This is where 60hps Mercy felt good and 50hps feels bad, because unless a 60Mercy had fallen behind on her healing due to bad priority and juggling, peeling was not something that had to occur for too long, if someone peeled they were able to return fast enough. Now, it takes a fairly noticeable time for someone to return after peeling, ruining the momentum of a fight. If you only have one support and it is Zen, you would notice having to peel when he doesn’t have ult. You would have to hide forever, Mercy is only 20 above that and it is still noticeable, enough that it makes the peel feel too long. Enough that 50 cannot provide someone enough time to peel.
Peeling for Moira is hardly any time at all, especially when you add after heal, it allows you to get back into the fight earlier.
Peeling for Ana is rather quick too.
But they can also sustain you far better in a midfight scenario which Mercy has lost most of the capabilities to do that.
These are important factors outside of the main issues of Valk and E Rez.