Ah I see, what you are describing is Merc stacking this is a known problem in pups and relates to every merc not only medics. Which you can see by the mirads of treads that complain about bushwhacker turret to strong (when there are 8), Skyhammer airstrike to strong (when there are 8), rhino minigun to strong (when there are 8), Naders grenadelauncher to strong (when there are 8), Artys Artillery to strong ( 
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The thing is this is simply a problem of people not knowing how to counter them.
Lets take your Uber team composition. While it might have the advantage of being unusual it is a bad one. You are stacking 3 fast squishies on top of each other and complement them with the slowest guy in the game. He will either be lagging behind all the time, or your team has to wait for him, negating your speed advantage. 3 of your team are best in stationary close combat while one sucks at it (Sparks). Also you got no engineer so repairs/plants/defuse will take twice as long (and you effectively loose a medic since he has to do that job).
Your big advantage is being able to revive people. However several abilities will deny that (Fraggers Grenade, every Firesupport AOE abillity, Vassili Headshots), Nader while not able to gibb (Martyrdom aside) can easily one shot the 3 squishies, take out the healingstation, and kill or soften up the Rhino leaving you with a Nader against a complete Team.
The second you lose the capapility to revive, either because all medics in range are dead or everyone that could be revived is gibbed you lose.
You brought only one abillity to the battle while the other team has brought, revive, the abillity to insta gibb several without a fight (there is a reason why competition only allows one fragger per team), survivability, firepower and most important the ability to clear every objective twice as fast. And thats the problem with every stacked Merc situation. Sure they are annoying and need specific tactics to overcome (which is hard to pull of in casual games, but thats a lack of communication) but a balanced team will always have all tools to overcome a stacked team.
No need to nerf medics over this.
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