Sometimes it's justified, sometimes it's completely overkill and you waste a lot of time. On a project I've been on I've seen days being wasted to build a basic account management form (think username, password, personal details, etc) in React for no reason, despite the same being achievable with our backend (Django) in a couple hours.
Our team has never felt more productive and the dev learning curve is much less steep. Clients are happier with the results, and the apps feel much more maintainable because there are less pieces to break / update / manage in the production environment.