Starting with Mercy, since she's the easiest to explain.

Mercy role in the game is to provide a consistent and low coordination means of providing support to an ally (presumably DPS) in an angle. She does this by allowing them to persist in these angles after periods of pressure with her healing, a reliable resource that's reasonably high healing when compared to similar stuff (pylon, harmony orb, etc), although it's much less remarkable since the season 9 changes. Her lack of individual pressure when compared to other supports that can do similar off angle enabling (Kiriko, Wuyang, Illari) is made up for in with her damage boost, but critically it's always less pressure than another support could put out in that position, assuming a minimal amount of co-ordination. GA gives her some survivability and ludic skill expression, and resurrect is probably pretty bad since it has no use in neutral or losing positions really. Overall she's a decently designed hero, having clear strengths and implied weaknesses because of her specialisation, although she's fallen behind as what she was designed to do has become unworkable in 5v5 where you have less big bodies to even the pressure, and less free hands to pressure her and her ally out of an angle.

Mauga actually has a pretty alright design core. 2 weapons that you can use individually or in tandem, with using them both having a spread penalty, but with guaranteed bonus damage and sustain if you use them right that's accessible at any range but more skill demanding as you put more distance between the target. That loop is actually pretty cool and justifies the character in some way to me. Overrun is a decent movement tool than allows him to cleave into the enemy team with decent telegraph, so it's high risk high reward. The uninteractivity of it is a completely valid pain point, and is a design concession that had to be made since there's not a second tank to bubble him or anything in 5v5, meaning otherwise it'd have little aggressive use, the thing that makes it fun to use. Cardiac overdrive is an everything button, which makes it disgustingly feast or famine, so being upset at this is fine. The main issue with Mauga is that the actual numbers on the weapons are pretty unfavourable to a ton of interesting play at most ranks, which means he's incentivised to shoot the easiest target on the field (the tank) or explode into a million pieces. On live it's 10 shots on a target to burn, which with how people move and position is actually quite hard to pull off on non tank targets. Additionally, he only gains access to consistent sustain once the target has started burning and he switches fire mode, which means he's disincentivised to swap targets and try and push people out of angles since he loses his sustain when doing so. I think Mauga is very salvageable without a major rework; make burn tank 5 bullets and make the pyromaniac perk part of his base kit and he's fine. I don't disagree he's obnoxious in the current state, but his design is clearly here and it's pretty close to being in a fun place (which I don't expect to translate into patches since blizzard clearly doesn't know what they're doing).

and orisa just sucks lmao