First of all: There is a bug with medic slots, max medic number is always set to 6.

Only for me medics are OP?
the ubercharge always charges too fast because the medigun is fast itself and also there is the syringe gun, it is very hard to avoid and gives 5% uber, nothing to say about crusader crossbow but the syringe gun gives too much uber for hit, in my opinion it should be nerfed to 3% or the fire rate sould be decreased.

From playing medic myself i know they can be overpowered most of the times.
This can be for example gridwalls (Dmg farming),Large groups of medics traveling together while the boss jumps in trying to kill them, they begin using the uber chain tactic, or the hale use stun rage in attempt to kill more than just one, but one was not stunned so he runs and activates uber to save them while the boss wasted a rage. (this can happen if the hale use the rage to others like engis or snipers, but is wasted due to medics switching target to uber).
The syringe gun=run backwards while firing at the boss to get fast uber.
While this tactic is cheap it can also be used to timewaste.

Nerf Suggestions
-Syringe gun=on hit. gain 2 or 3% uber.
-Reduce the medic limit to 3 or 4.
-Medi gun=while ubered, switching healing target will decrease the uber% faster
-Reduce the heal time while ubered(medic while uber gains overhealing on himself).

I think this stat already applies in regular TF2. But maybe also start uber percentage from 0 instead of 42 or whatever it currently is.