A) makes you fly, giving you much much higher mobility.
Flight could be nice but the current version of it in Valk has problems.
The flight speed is slow which at higher levels where hit-scan players are good means you’re dead if you show yourself. Mercy glows like a beacon so she’s not hard to miss. GA is faster than flying.
The activation of the ult isn’t fast enough to avoid enemy ults so it’s not great as an escape mechanism unless you’re already flying. However if you’re already flying then you’re either in the fight being Battle Mercy so take the risk of being caught like any other DPS or you so far behind your own lines that ults won’t catch you.
Having flight removes the quick thinking and positioning needed to stay alive as Mercy though the use of GA. You don’t need to manage your position vs the team vs the enemy vs the environment as you can hide in the sky box. It promotes a duller less risky style of play and removes skill from the character.
The extended range of chain beams means flight is perfectly suited for sitting far back behind your lines and healing. The best thing it’s good for is Battle Mercy to hunt snipers and dart around in the team fight murdering low health targets. However that means you aren’t healing which is your primary task.
Mercy is supposed to be a healer of paramount ability, the best in the game. Having flight as it is actively promotes Battle Mercy which goes against her character design, even if it is the most fun thing about the character at the moment.
B) changes you from single target hero to AoE (or chained).
Chain heals removes an element of skill from the character. Namely juggling your healing from team mate to team mate to keep them topped up in the fight.
The radius of chain healing is small, about the same size as Lucio’s Amp It Up and does a little more healing. It’s not good enough to counter Ults or sustain your team in a fight. It’s not enough healing to be ultimate worthy as it won’t save anyone.
It also goes against her character design as a strong single target healer. Their are plenty of other characters who can AOE heal. Mercy doesn’t need to be like them, she needs her own thing.
C) actives your self heal for the full duration of the ult making you harder to kill.
Self heal is nice but only relevant when you’re playing Battle Mercy. With the length of the chain heal and flight you don’t need it when performing your primary function, healing. You’ll be sat up in the sky box watching the fight.
D) gives you infinite ammo In case you decide to do some pew pew.
Good for Battle Mercy again, not very good in helping you heal and fulfilling your primary role.
E) (this is still fresh but still) increases your healing.
I don’t know of any healing increase. She still does 60hps which Mercy has done for years other than the time she was reduced to 50hps.