Complaining is hard.
Well, usually if we're complaining, it's because something isn't done to our satisfaction. We often take it personally: "How dare they treat me so poorly by not delivering as promised!"
We get made. Our natural reaction is to start slinging blame. The reality is that does little to solve the problem.
It often causes the opposite because both parties start acting emotionally rather than rationally. And that never goes well because everyone tries to prove themselves right.
I suggest to people that they instead base their complaining on two things:
The facts. Just the facts. If you're complaining is justified, the facts will back you up.
Niceness. Over the years, I've learned how to just nice people to death. "Well, I know you guys tried so hard on this but it isn't the way you promised."
I've found this is the most direct way to get complaining to turn into results -- and that's what really matters in the end.