There’s a TLDR at the end
I personally disagree on some of the statements.
I don’t believe that one-tricks inherently are anti-teamwork-- many one-tricks actually require really good team-work. The main issue arrives when they are unable to adapt the team’s compositions, resulting in a loss for the team. You could argue that the team should adapt to the one-trick, and I agree with this train of thought. But when the one-trick is also unable to explain why in a concise and logical manner along with a plan (ie. Sym. as a DPS w/ a Mercy and Moira, where Mercy is pocketing Sym) is when the one-trick becomes anti-teamwork.
Part of teamwork is explaining a plan, and the idea behind it, and accepting when the team doesn’t adopt your plan and adapting to that choice.
Another example of what I consider anti-teamwork is when the one-trick demands the team play around them. For example, semi-recently I was playing with a one-trick Sym. Now, I had no issue with this. We started on defense, and they demanded the Mercy pocket them. The Mercy didn’t, and kept the tanks alive because the Sym didn’t fit our composition. The Sym played as if the Mercy was pocketing, and got rolled. This led to our complete loss. Now on attack, I tried to (politely, in my opinion) point out that Sym isn’t good on attack (There are many scenarios in which this is untrue, such as barrier+turret pointed out by a response. I don’t wish to perpetuate false information, and I do admit this was more than slightly salt-driven at the time because of the entire lack of team-work the Sym was showing). They denied the statement in it’s entirety (Edit: They did say that they normally had a team to work with them, and probably just assumed everyone knew how to do this comp). This is anti-teamwork, and why the Overwatch community believes one-tricks to be anti-teamwork.
I’m sorry if the end seemed like a bit of a rant, it’s completely unintended.
Some one-tricks are anti-teamwork, which paints the entirety of them in a bad light (even if it’s unjustified in many situations).
Teamwork is working with the team, not going off playing your own way (this isn’t a jab against people who have few mic communications, there are other reaction based ways of working with your team such as seeing your Rein charge in and nano-boosting them as Ana or shielding them as Zarya)