I used the JIT version recently on a new landing page with code completion for class names in my IDE and found it great. The distance between what I'm picturing in my head and what I have to type to see that on the screen felt so much shorter and less fatiguing than the standard CSS approach. With the regular way, to style something that's probably not going to appear elsewhere, I'm having to: come up with class name…
The trick with CSS is to write semantic HTML and avoid div, span and css classes. You can of course inline css too (used mostly for optimization to prevent layout shift, but is also fine for elements/classes that are not repeated, such as the top menu, top banner/intro and header/footer)
This experiment has failed though. CSS isn't powerful enough to style HTML however you want without having to add a soup of extra divs and classes that are only there for styling. No large website today works otherwise.
HTML is still semantic when it contains styling markup (in the sense that a computer can read and understand the structured data from it) so I'm unclear what benefit is being missed out on. Whether you call a class "home-cta-subheading" or "text-center" so you can apply some styles doesn't make a difference to screen readers, browsers or search crawlers either.