I do not agree with other comments : I'm happily using Svelte in a very specific webapp with a lot of pages : forms, validation and datagrids with complex search queries. I use zero external UI components : if something missing, I can build it in Svelte quickly.
It's simple, predictable and ... just works.
TLDR: Svelte is like Rails, lots of magic to do pre-ordained stuff quickly. If it can do everything you need your UI to do it will be very productive. But if you have UI requirements that don’t exist in Svelte (or Sveltekit) out of the box, you may be better served by a more primitives/toolbox-based framework like Solidjs.