- Eliminates weapon variety: Its so good that all the other primaries are considered "trolling"
- Stand-still meta: Nothing says "fast gameplay" like seeing half your team standing for arrows in the middle of a fight.
- Viewlock: While medigun lets you look around while healing, crossbow forces you to place your crosshair at your teammate and lose situational awareness, wich might stop you from seeing a soldier jumping you or some scout flanking for you.
- Gameflow: This isnt related to medic specifically but to the entire team. Even if they dont stand still for arrows, the midfights and teamfights in general will get drawn out longer than usual due to the crossbow insta +100HP. "Demo is wea- oh nevermind arrows exist" This forces to play using damage bursts, as leaving someone weak doesnt really mean anything anymore. "Bow me doc" and instantly back to full health, woah nice skill you didnt dodge your med's arrow.
- Fun? Thats subjective, and let me tell you buddy, i dont find this stagnant clutchbow meta fun. Back when it didnt build uber and nobody stood still for arrows it was more fresh and fun to use but now that you're expected to pull it out every 4 seconds its nowhere near as fun.
One might think "but it heals so much faster so it makes for faster gameplay", but its actually the opposite. Everyone lives longer, everyone takes more damage to kill, and worst of all, people hide away or stand still during fights to get their sorry ass back to full health.
From a wider perspective, even if some medics say its fun, it also sucks out the fun for the rest of the players by just making fights more frustrating and drawn-out that they should due to its huge healing cappabilities. Its basically rescue ranger for medic after all
So its not really about the crossbow being OP compared to other medic weapons, wich it obviously is, but its more about how it harms game flow in general.