If you don't have anything nice to say...
I beg to differ. If I consider a piece ill-thought out crap and can explain why I think it is ill thought out crap, then I think it appropriate to say so. If no-one were ever to point out flaws in work, the author is never going to improve.
There are different ways of phrasing the criticism, obviously. Harsh criticism can and should be presented in a constructive manner. But if one goes with the phrase you use, then it's just hug-box time and everything is lovely. I would have far rather had something like: "Your depiction of a teenage girl is really unconvincing. She's acting either as though she's far more mature than her years might suggest, or she's acting as a child rather than a teenager" than: "That's really good."
As examples, several people in various places have written elements of stories that go into some detail about infantry combat in modern(ish) wars. The majority of these have been so clearly written by authors who, to put it mildly, haven't got a clue about how people think and act and react in combat (Keynes' Cruisers is an honourable exception to this, and is, in my opinion, brilliantly written). It's a subject area I know a bit about. Should I be silent when people get it glaringly wrong?