If you are extremely disciplined when it comes to your custom css, sure its better.
But like any framework or library, tailwind lets other people jump in quickly and be productive because its a standard.
Though i may be biased because i love using tailwind
I understand that you can start creating faster using their utilities, but as a UI library creator, I have to write more complex styles and can't do this with Tailwind.
Would implementing the utility first approach of Tailwind in Sass be better i.e. a Sass interpretation of their @apply directive? Packing a bunch of class names into HTML doesn't seem like a great developer experience.
None in my opinion. Introduces bloat in both the css and html. The BEM pattern is far superior.