I have just been reminded of one of my standard lines to another comedian after they have gotten off stage. You have to be careful as a fellow comedian and sometime stager of shows. Giving feedback can be a double edged sword. I think it is massively important to be critical and to wonder how things work, whether they really work, and how to make them better. I remember reading Frank Skinner's book and losing loads of respect for him as he clearly didn't have a clue how the world, or the art of stand-up, actually work. What Skinner has is good instincts and stage craft, but not by design.
Giving feedback can really help someone improve, and I think there are a few acts out there who would speak well of nuggets of feedback I have given them which they have been able to use to good effect. But nobody likes an opinionated wanker who tells them what they should have done differently, and I made way too many mistakes in my early days as a comedian giving opinions that were not asked for, welcome, or considered to be authoritative given my own apparent experience or level of skill. It doesn't matter whether you are right or wrong if you are giving feedback inappropriately.
Feedback to comedians is even more of an issue in general, though. Stand-up is ALL about the feedback. We speak, they laugh. That's your feedback loop. Simple. If a comedian gets the wrong sort of feedback, their ego implodes and the defensiveness begins. On top of that, all new comedians are, on the whole, not very good at it. They will get better, and there is no point in making them feel suicidal by telling how much they suck while they are still learning.
So if I have really loved an act, I will tell them. I will tell them why. I will gush and ooze and it will be a bit gratuitous, but they will probably not hate the feedback because they will, deep down, be getting what they asked for. If a fellow comedian does a good job, then I will say it was a good job. If someone I don't know sucks a bit, then I will avoid giving them false positive feedback - I don't want to reinforce their belief that that shit is good - but I will spare them from my actual opinion. I will, instead ask a question. It's usually "Did you enjoy that?". I choose that question because I firmly believe someone should enjoy a comedian's work, and if they didn't enjoy it themselves, even, then they will probably spend time working out why it wasn't enjoyable, and will fix it.
I enjoyed writing this.