It still helps deduplicate.

Only if you are using a framework like React, where you create generic Button and other components.

Try using Tailwind with server-side rendered frameworks like Rails or vanilla HTML, and you will find yourself repeating the same stuff over and over again. Basically anytime you want e.g. a <button>, you have to repeat all the inline css for it. (There are plugins for Rails that adds "components", but I found it very clunky)

Not really. Every single button that gets rendered has thirty class names attached to it instead of one. It really bloats markup with duplicated classes.