Jack of two trades, master of neither.
Yes its possible to do both, but short of the intellect of Isaac Newton it is absolutely not possible to be excellent at both.
I am an excellent back-end developer; that’s not chest-beating, it’s based on work quality and productivity and the fact that I have managed four servers, database and schema, and IIS at a rapidly growing company, working remotely AND handling site emergencies at all hours. I know just enough JS to touch up call points without bugging the browser developer with the communication load of incremental changes.
I like that division of labor, that’s my idea of a functioning group. Nobody else messes with my code and puts in clutter or early returns and I don’t do guesswork in the JS and HTML. I do know enough to find bugs there but I let the browser guy fix them.
Full Stack is a job description by people who don’t want to pay two people. They get what they pay for: mediocrity at both ends.