Many us have been on sports teams and part of other types of teams. "Chemistry" is real but it is also a very general idea. For example, relative to basketball, there is on court chemistry and off court chemistry. It has been proven enough times that you don't need to be best buds off the court in order to be good together on the court. To me, the on court results have more to do with buy in or acceptance.
There was an example above of Kyrie and his time with LeBron/Cleveland. On that team, Kyrie understood and accepted a certain role. LeBron set the tone and players toed the line. The team was successful so no one complained about chemistry.
Kyrie on the Celtics was a different story. Kyrie never really figured out his role in terms of on court and off court leadership. The result was what appeared to be bad chemistry but I don't think the players liked or disliked each other any more or less than the players on the Cavs.
I think the chemistry will be fine, sufficient, for the Nets. Durant will help with that. We'll see what happens if/when the team struggles and how the relationship between the coach and players holds up. To me, that is the risk but normal for an inexperienced coach.
I think the chemistry on the Celtics will fine, even good. There may be dust ups along the way but to me, that doesn't matter. I think players will buy in, accept the leadership of the coach and the top players just fine. By and large, I think the players are happy to be here and happy with their roles on the team. That is a good start to good chemistry.