I believe that if you are not hard enough, you will never make it. Therefore, I always try to give the most productive feedback possible. If you show me something that is incredibly boring and I think it's not going to make it, I'll say so. I always preface my arguments by saying that I'm no fortune teller. And in the end, I might not be that good at understanding what readers want.
But I still find that some books are clearly written so badly that it's easy to tell people how bad they are. When a book is written decently, it's much harder to predict whether or not it's going to be a success. If someone paid me an insane amount of money, I'm confident I could give an estimate of how successful something will be. But it would take hours upon hours of line-work. And I don't do that.
I am firmly against Toxic Positivity in Writing and being too nice to people, especially when they are doing something like writing. I do also believe that If you enjoy the process, results will follow. But enjoying the process doesn't mean that you just avoid the difficult questions. It means that you embrace them in your life and use them to course-correct your process. Often, people think that course-correcting is an unthinkable act against nature; that whatever you are doing, just keep going girl! Bleah. I'd be immensely surprised if anything coming out of an amateur writer is even remotely entertaining: that's why I'm often too blunt with feedback.