There are plenty of things to be told by “friends” who are complaining together. I've had a lot of bad experiences too, so I can say for sure. I also have to be careful going forward, and don't let your guard down either. Also, since it is a well-known fact that lines and emails are viewed by third parties, you are probably familiar with it. So, from this point of view, it means that complaints should not be made.
Next, let's consider whether it's okay or bad to complain from a different perspective. In conclusion, complaints come out of nowhere, but I think it's better not to raise them.
My husband and I, the matchmaker, spoke loudly to me on the night of the funeral of the former chief priest of the temple to which I belong, “If you praise Nembutsu, anything will bring joy.” However, he is a very grumpy person. I'm an adopted child, so when the former chief priest of his temple was alive, I was always upset about my father. My father-in-law also seemed to be complaining, so he was often upset about adopting children. They both went out to talk bad about their adopted son and father-in-law, so their bad relationship was well known among the monks of our sect. If you look at this, you can see that complaints come up out of nowhere, and it is difficult to stop them. It's like this even when praising Nembutsu.
So, when it comes to whether it's okay to keep on complaining, I don't think that's the case. That doesn't solve anything. There's no way the coach will change even if they complain behind the scenes. Also, it is certainly possible to vent by complaining. However, according to the Buddha's teachings, that act only strengthens the mental tendency to want to complain more and more. In other words, the more you complain, the stronger your tendency to have a bad personality. The father-in-law and son mentioned above were complaining about each other in the world, but I think what was easy to refrain from at first escalated steadily. Since people with different home environments are adopted and enter the temple and become family members and peers, in reality, complaints to a certain extent are probably unavoidable.
The Buddha took the position of staying away from (moving away) from the source of complaint and not complaining. It seems that they valued staying away (staying away) from things. That's because it's hard not to complain.