It's possible I haven't hit any of the downsides yet. With that caveat out of the way, I will say that I absolutely love it after using React for about 5 years. Here is why:
1. It's simple. The API can be read and fully understood in a day or two. The documentation is clear and their examples are simply wonderful. Seriously. Go through their tutorial (https://svelte.dev/tutorial/basics) or their examples (https://svelte.dev/examples#hello-world) or even their API docs (https://svelte.dev/docs). It's all well-crafted.
2. It has CSS modules built-in. You don't need to install anything or configure anything. You insert a style tag, write some CSS, and your component gets style without stepping on global styles.
3. It has state built-in. It has a dead simple state protocol. You can even recreate a redux clone in about 20 lines (https://monad.fi/en/svelte-custom-stores/).
4. It's compiled, which means it can do things other frameworks can't. Which also means you get actual messages from the compiler telling you what you are doing wrong and what you should be doing. The messages I've seen so far from svelte are easily some of the best compiler messages I've ever seen.
5. Minimal edge cases. There are a few edge cases (e.g. arrays and reactivity), but it's fairly easy to know when you're hitting one.
6. Way more flexible than React. Event forwarding, event modifiers, boolean class names (i.e. class:large adds the "large" class if "large" is bool and true), ability to create generic actions that can be added to any DOM nodes. Animation support built-in. Variable binding, makes it a breeze to create any form.
Sapper is going away, but SvelteKit is being worked on. If you really need more than Rollup+Svelte.
They are similar in a sense that both compile HTML to incremental DOM-like structure, but have their own differences.
Haven't heard anything about Svelte in months / years, although usage is steadily increasing (https://npm-stat.com/charts.html?package=svelte&from=2017-11...).
Im not 100% sure how it'd look, but I reakon with Kotlin Multiplatform you could have a full stack application, all in one multi-module project, all in the same language, in a language not dervived from Js.
One to aseess when I get some time.