Team Fortress 2 when played at a competitive level is a game of forming advantages and snowballing those advantages into momentum.
Advantages can be anything
a pick advantage
a positional advantage
a uber advantage
a damage advantage
The Quick Fix does one thing good outside of midfights.
It kills advantages and prevents advantages from snowballing into a successful push.
It is insanely hard to kill a aware QF medic with anything short of kritz while he is backed into a corner. The QF jump mechanic is far easier to use defensively then offensively.
Advanced mobility causes stalemates? You mite ask but when paired with the context of the QF it does.
The QF doesn't deal well with focus fire, which makes the tethering difficult to use for aggressive play, and the uber itself can have real trouble pushing against a prepared defence because it lacks the resilience that lets a stock charge bludgeon its way through (and it can't win by alpha striking unwary defenders like the kritz can).
The flipside is that the overheal + tethering is really damned good at forcing a pop and then either bugging out entirely or kiting the rest of the uber. This usually leads to a failed push as the other medic will have his QF uber soon and will simply retake the ground you covered.
As for damage advantage? Well this is self explanatory. Playing against QF is like playing against a team where everyone constantly has crit heals. Anny spam damage you do is negated faster then you can reload to take advantage of it.
Uber advantage means little in the QF metagame. The uber advantage itself is not enough to viably push. You need both a number advantage and uber advantage to push QF vs QF. If you do not have the number advantage you will not be able to bully them out and with the QF they will simply force you, run, then fold back on you with there QF uber (they will most likely have to pop it to get in do to your positional advantage). The process then begins again. While conventional uber will get you though the chokes far easier and allow for more agro use of the uber yo will not be able to catch a QF combo out because they can simply hold close to get the early force then jump away. THis leaves you in a position where you have to run the QF our be at a constant uber disadvantage.
(to be continued)
Partial credit to Hypnos