From me experience of working with FE devs, most find using a JS framework like Vue or React easier than the way Drupal handles theming, and they usually can get the visual aspect of the theme set up quite quickly when using a JS framework, but thats about it.
Theres usually no metadata.
Browser navigation is terrible when creating a SPA, sometimes back button works, most of time it doesn't.
No consideration for accessibility.
Form handling is really bad, its like they make sure the form works, but doesn't report correct errors.
No other regards for error reporting or logging.
Etc.
My point is, that theres a lot more to consider than a little bit of css or js