TINSFV
Balance: I think so. The fast heal rate and uber building is strong, but halving the buffs and not granting invulnerability are sizable downsides. You also cannot block point capture with the uber. Also, it is useless against a Kritzkrieg.
It is better. The Overheal nerf is not significant because the extra buff is not enough to initiate a team fight against the healing, and the Quick-Fix allows Medics to disengage from poor fights incredibly easily. To expand upon the first point, in a no-Uber situation all the QF has to do to break even with stock is to ensure that the fight does not end in 7.5 seconds...ish. It's weird because the classes playing near the Medic will stay buffed, but that's the point where Overheal decays to the 50% mark, and once that advantage is gone you have nothing. You have to get your team in their faces in that time frame, which is very difficult, because, again, the Quick-Fix is much better at disengaging from shitty fights, because you can jump. During the Quick-Fix meta, Ubercharge was only run (once people besides Mixup figured out it was broken) to counter Kritzkrieg and push vs turtle classes (which were much more frequently run due to the turtle nerf).
Not granting invulnerability is one of the shittiest things about it. It is bad at pushing through chokes, which means it's bad at pushing, which means nothing happens. (which is bad for the metagame because doing nothing is boring and pushing is fun)
But it's only a downside against a team that knows how to counter it. In a scenario with standard classes where one team has stock and the other has QF, the QF has a massive advantage - it builds faster. What the Quick-Fix team needs to do is trade Ubers. In a no-Uber situation, you will win because of the heal advantage. Or you can just build and use your 8 seconds where they don't have Uber to kill everything. Also, if you fuck up, just jump your Medic out, you'll have Uber ad and can try again.
Skill: It adds the dimension of Medic latching onto a Soldier or Demo and flying across the map. Conversely, the Medic must take care not to be jumped in inadvertently by his heal target. This increases a Medic's need for awareness and knowing his team. The nature of the Quick-Fix uber forces teams to be very aware of sticky traps when pushing through chokes, especially into last. Like the stock, the Medic must know whom to flash with the uber.
There are only three difficult things about 6s Medic.
One of them is arrowing people. (EDIT: oh yeah i didn't think of this before, the quick-fix lessens the need for this skill because it builds extremely fast and has offers more zero risk healing vs the high risk high reward of attempting to arrow a teammate)
The second is Pyro DM.
The last one (and only one that really matters) is positioning. Positioning is one of the only facets of Medic that takes skill (and you don't need it if you are getting micro'd, Medic is also the easiest class to micro). The Quick-Fix removes this facet because you cannot be punished for bad positioning, especially since Medic shoots further than the actual Soldier he's attached to (and therefore a potential Soldier that's chasing him) due to his lighter weight.
Also, you say that flashing QF takes skill. It does not. Flashing Uber takes skill because you need to decide if they are GOING to take damage, which is often a difficult decision. Flashing QF does not take skill because you only need to flash megaheal to heal things that are already hurt. You don't need to think, just look at their health bar.
You have to be aware of stickytraps regardless of Medigun, including Vaccinator (you need to pop to save people from getting killed by the trap).
Play with: If it's not fun to play with, no one will use it.
Lmao, what are you talking about? Competitive teams and players don't do things to have fun. The only thing that matters in a competitive game is winning. People will use things that allow them to win. Sitting around your combo as a Heavy / Pyro in HL and spychecking and getting 5 kills and 50 dpm in an hour long game isn't fun. People still do it because it's how you win. Playing 6s Pyro or Spy is fun, they're still worthless garbage joke classes and that's why nobody plays it or wants to pick up a Pyro or Spy.
Play against: Since I have minimal experience in a competitive setting, I won't try to answer.
A point-blank Heavy has the DPS to crush megaheal.
Sniper hits through the faster heal rate / megaheal.
Stickytraps go through megaheal.
Axtinguisher used to.
A very good Kritz punches through it (if it's a glancing blow they can survive and surf away).
there are a bunch of other things like jarate and phlog and spy and gunslinger crit punch but the former is blatantly broken and the others are obviously jokes
Do we want a game where Snipers stare at each other for 5min?
Do we want a game where Heavies are run outside of last?
Do you want a game where the team that pushes is always the team that loses? Because the Quick-Fix is amazing at defending because all you have to do is draw out the fight and you will win, and it's very difficult for your Medic to die since he has such a good escape tool. It's also shit at pushing through spam, traps, and Sniper, which means much more waiting around for a Sniper pick.
Also, Quick-Fix was unbanned before the Axtinguisher was gutted and (more importantly) before stickies were nerfed. Kritzkrieg is much shittier now.
From the standpoint of casters and spectators, the Quick-Fix makes for a rock-paper-scissors game of Medi Guns, where Stock beats Kritz, Krits beats Quick-Fix, and Quick-Fix (used properly) beats Stock.
FUCK rock paper scissors. Rock paper scissors is NOT what makes TF2 interesting.
One of the many reasons that Overwatch is a shit game is that it is full of hard counters and rock paper scissors. Pharah is deleted by Hanzo or any of the hitscan heroes. Reaper is absolutely, 100% worthless against Dva. Zarya existing makes Junkrat and Pharah useless. Tracer 1v1s everything. Yes, none of those characters are necessary overpowered because there is counterplay in a team setting - but there is nothing as frustrating as seeing your hard counter and knowing you may as well take your hands off your keyboard go take a piss because you chose wrong.
The reason 6s is fun is all the combat classes have the potential to outplay each other, and the format creates opportunities for them to do that. That sort of thing will never happen in 6s unless you play Medic or Pyro, or are playing against a very good Sniper, or are playing Scout vs Sentry + Engineer.
The reason the Quick-Fix meta wasn't fun is that what allowed teams to win wasn't being able to win those exciting team fights, but switching Mediguns and exchanging fast, mobile classes that are equally good at pushing and holding to slow turtle classes that are trash at pushing and can easily punish attempts from the enemy team to initiate fights, leading to a whole lot of jack shit going on. Also, Kritzkrieg is trash with the Demo nerf so maybe it will just be Quick-Fix vs Quick-Fix with Ubercharge to push last. Which is also shitty.