You know, I don't really think that it is. I'm only speaking for my own personal experiences when it comes to this particular phrase, but at the end of the day, "just be yourself" is fantastic advice, if it's actually taught in a meaningful way.
We have to teach our kids as they grow up what "yourself" actually means. Instead of carving out what we think it means, we should be giving the kids the tools and knowledge they need to be able to define that. And when they end up on a path that seems "different" to our ideals for them, we should intervene only if it's a path that will be dangerous for them. That's what we do as parents, right? Guide and protect?
I grew up in a home where "yourself" was defined for you; we were a strict Mormon family, so a great deal of my path was already regimented. But when I got out of the Mormon church and met my wife, the phrase "just be yourself" meant so much more, and was immensely freeing. "What is this 'myself'?" I'd thought. "How do I define it?" And then she helped me explore and discover who I felt that I was, and what made sense to me.
As such, I want to raise my kids in a manner that keeps them curious and exploratory. Teach them to research so that when they have a question or someone poses a new concept, they have the tools to figure out what that concept is to them, and formulate their own opinions. The question I want them to ask more than anything in the world is "why?". Then, with all the information in hand, they can answer it themselves.
"Just be yourself" is cruel and fraudulent only if the parents and influential figures in a child's life has set them up such that "yourself" is "how you're supposed to act according to me", and not something that was genuinely fostered by the child, with gentle mentorship from their parents.