They specifically mentioned the liberal establishment. You're talking about criticism from people that probably abhor the liberal establishment even more than they do progressive liberals like Bernie.

Also I think this kind of criticism is important and I don't know why it bothers people so much. It's okay to be critical of things you ultimately support, either for ideological or simply for tactical reasons. It's called critical support, and I think people should do it more often. Even if the criticism isn't ultimately supportive, that doesn't mean all of a person's hate is directed in that single place. There may be more than just the surface level WHAT, like the WHY of it all and what that implies, that you are missing (or dismissing).

You have to stand for something or you'll fall for anything, and refusal to engage in critical analysis - pretending any politician can do no wrong (or the contrary case; can do no right), getting defensive, and outright rejecting any investigation to prove or disprove your conclusion - does not fall into the category of 'standing for something' to me but rather overzealous team sports.

We have to practice more critical thinking, despite how badly our political class does not want us doing that. Whether it helps any specific politician win an election or not (which you can still do even with criticisms). Especially considering that it's this kind of criticism that has made it untenable for a growing number of politicians to deny the genocide in Palestine; it's pretty clear that the only needle that uncritical support will move is that of the progressives, towards the liberal end of the spectrum. After all, it's our criticism of the current system and its complicity in human suffering that makes us progressive in the first place.