Tailwind's default sizes add constraints, which makes Tailwind a bit of a design system. Also, Tailwind's default colors are just very pleasant to work with. Even on projects where I don't use Tailwind, I look up the color table in the docs.
Same, I really have nothing bad to say about the Tailwind design system.
I suspect there is a lot of halo effect going on, where if people were asked to judge the pure code aspect of Tailwind vs the design system separately, they'd be much more negative on the code aspect of it, but because the two are conflated and one is very good, Tailwind as a whole is judged to be much better than I personally think it deserved to be. It doesn't help either that forming an opinion on an architecture of code is difficult, and forming an opinion on a designed web page in front of you is easy.
My personal poison is just defining the Tailwind color system and a very basic system of sizes in CSS variables and then using that all over the place. It leaves an escape hatch in place of just using 5px when you really need to, but most of the time we use var(--col-grey-200) and var(--u-4) or var(--u-8)
