I didn't get the change to customize the default and built-in components from Clerk. So, it's hard for me to share any experience. But, I know you can do little customization using https://clerk.com/docs/components/customization/overview and play with CSS and Tailwind CSS. Maybe, for complex customization, it's preferable to not use the pre-built components and build it yourself.

Currently, I just feel the default UI from Clerk works good with Shadcn UI, both works great together. Definitively not perfect but do a great to job for a solo developer or for a small team.

And, I mostly using for advanced authentication features like user impersonation, multi-tenancy, roles & permission.