The Ubersaw is not OP, it's just not actively bad
Medic has 5 options at melee.
The Bonesaw (Fine) / The Ubersaw (Good) / The Vita-Saw (Actively Detrimental) / The Amputator (Irrelevant/Useless/Weak) / The Solemn Vow (Powerful/Useful)
Think about it for a minute. What is a Scout's most powerful melee? The Atomizer. Triple Jump makes for huge mobility buffs, is always useful in and out of combat, and you generally want to utilize it as much as possible.
What about Solider, (You have some options here) but the Disciplinary Action helps speed you out of spawn, has a high range to catch enemies at distance, and if you're not actively reloading or shooting something you can buff your team to help their combat ability.
Heavy, Fists of Steel, if you're ever caught out of position, flanked, in danger. you just pull out the "please double my health" melee and leave
I could go on, but there's something funny about these melee weapons. if you had them on Medic, would you still use the Ubersaw? Really consider it, If you had an Atomizer that gave you a double jump and let you evade damage to escape, wouldn't that be better than having to risk your life trying to hit an enemy to gain Uber and likely dying?
What about the whip? Wouldn't it be absolutely disgusting to whip your heal target right before an uber or kritz and watch them go super speed into the enemy?
and the Fists of Steel, I need not even explain how a 300 health medic is basically impossible to kill as long as you're not trapped somewhere, and even if you are. That bolstered random crit rate from healing would serve you pretty well if you were basically invincible due to being at minimum 3 perfect shots to kill for most classes.
The Ubersaw isn't something you intentionally pull out to accomplish your goals. You pull out the Ubersaw because the alternative is that you die, just like any other Medic melee, and from a practical standpoint, the Solemn Vow is objectively better for the goals of Medic assuming you can communicate with your heal target (Or can aim the crossbow at wounded enemies). Hell, even if you can't communicate, a savvy heal target can see you shooting a crossbow at something and understand that you want him to go kill that because he's low health.
Fact is, the Ubersaw is fun on a visceral level, not a practical level. A joyless robot uses the Solemn Vow, F2P's use the Bonesaw, bad players use the Amputator, Bioshock fans use the Vita-Saw, but you use the Ubersaw if you're the kind of person that likes to inject dopamine directly into your veins, and you're allowed that luxury because there's nothing that's far and away better than stock in-slot that forces you to change, not because the Ubersaw is too strong, because everything else sucks or is boring.
Sometimes, that's a good thing for the fun of it all.