I honestly think mercies are fine for everyone but Cybran players. Sky slammers cant do fuck all vs a mercy due to their terrible projectile speed. Regarding mercies, Its a fairly huge investment that takes time to pull off, and I think in most cases equal mass spent in t2 gunships/figher bombers will yield the same/better result. I do agree that the gameplay mechanic is poorly thought out and not fun, and could use a change. But I disagree that they are OP.
IMO the only time mercies are a problem is in huge teamgames when players aggressively push without scouting, AA, or air, and that's mostly their fault for doing so, especially if they see enemy air player is aeon.
That being said, I would be onboard for changing the role of the mercy, or even tinkering with it to make it less 'feels bad man' when you get sniped. I would support either lowering the speed of the mercy, or keeping the speed the same but making activation radius super low, making it so that AA even with/slightly behind ACU can stop mercies from killing ACU.
Someone had a crazy idea of making the mercy behave like a dumb TML missle, which sounds pretty cool if it could be balanced. Buff HP, make it not able to track units, but weak to TMD/AA, and use it to snipe stationary targets.
TLDR: I don't think mercy is OP, but I think its a bad unit that could benefit from a rework.