It's super comprehensive overall, and I particularly like the documentation for the components. For example, even for a basic Checkbox (https://polaris.shopify.com/components/forms/checkbox), they provide sample code, accessibility guidelines, dos and don'ts, and related components. They have clearly put thought into how the system should feel overall; design systems are not just about the CSS of the UI pieces.
It's also cool how they display prop types; if it's a more complex type, then you can click on the prop for details, and click even deeper if needed.
Tailwind CSS makes it super easy to style any component and completely side-steps the issue of well-structured (and inflexible) CSS systems.
Tailwind UI fills in the "bootstrap components" gap with a bunch of premade templates, which are very easy to alter for your own, highly-custom use cases.
The UI kit is super expensive though. I'd need to pay $450AUD for the complete set of application UI + marketing UI templates ($370 if bought together). For an individual, it's borderline obscene, but the pricing makes sense for teams.
This setup is super useful. I really enjoy it, though it was a struggle to justify as a student.