“Just be yourself.”
If “be yourself” means “be honest about yourself, your weaknesses, and your abilities, lie neither to yourself or others” then I agree. If it means do what you truly and really think is best, then I absolutely agree.
But what it really means is something like this excuse your excesses, wink at your weaknesses, befriend your faults, and ignore your ignorance.
If I were to tell a struggling Greek student, “just be yourself,” then they would remain a non-Greek knowing person.
If a new lifter goes to the gym and acts like themselves with the weight equipment they will either plateau at a non-optimal state, injure themselves or become a gymbecile.
How about a Christian who is addicted to various evil behaviors? Should he simply “be himself?” Or should he try really hard to discipline those habits out of himself?
Anyhow, I think the advice can be stated in a useful way, but in the romanticized way it is commonly used it is a bad and dangerous idea.