Here's some thoughts I was initially going to post here
But finally, I consider it was worth starting a new thread.
No matter how visible (and all the mechanics associated with it) Phantom has been throughout the last iterations, he remained a sitting duck in any properly set environment. Even at the total invisibility status we had at the hunter release/phantom rebuild, I still don't know what place he could possibly take in any 5vs5 situation.
And here is a massive issue SD has to deal with many mercs in this game : half of them are just useless or a worse pick than something else when merc choice turns important.
Between public and ranked/comp, there are so many balance discrepancies, they'll always end up saying "50% are happy with the merc, 50% not happy with it". And sometimes the voice from comp/experienced players is the literal opposite of public ones. Proxy just to mention a notorious example, which still awfully struggle in competitive setups.
In my view, those are the merc which are highly valuable in serious match setting, and how they seem to fair from a "pub" standpoint :
Medic : Everyone but Guardian has its usage, Sparks being a bold choice and has to be played very well (I'll skip on medics, that's probably the most balanced class so far)
Engineer : Fletcher still remains the best engineer in my view, with turtle having some place to shine. Proxy and bushwacker are quite behind. Interestingly, in pub, proxy shines while fletcher is meh.
Support : Arty, and eventually Stoker, especially combined together or with a nader.
Skyhammer is half-assed with his cooldown, kira too weak, and javelin a bad remix between support abilities and fragger. We all know how dreadful a javelin stack can be in public.
Recon : A nicely used Red Eye will always seize the cup in comp oriented setups.
Vassili and to a lesser extent Aimee can perform well too. Phantom totally non existent. Which is literally the opposite of public games.
Combat : Thunder, Fragger and Nader.
Fragger is turning quite hard to play in public, catching the strong focus on him with only +10hp compared to supports, but remain very potent in serious games.
So here is the thing : as long as SD is unable to make any merc an interesting choice for both the competitive and public side of the game, there is no way they'll end up to something acceptable. Most of the significant tweaks or new merc release we recently saw came from testing by comp oriented players (PTS to name it), in a competitive setup. But the result is generally quite far from expected in public - Javelin being the biggest example.
In a nutshell, as long as they can't merge situations, for both public and competitive players, the balance will always be a nightmare. And its definitely not a phantom, javelin, or whatever merc problem. Its broader than that, and has to be treated as such.
EDIT : Some additional food for thought which is induced by this wall of text :