It's cute but I can't recommend it for a project with more than one developer. Lack of TypeScript support and the hacky component model (specifically, Svelte's equivalent of React props) are the two big deal-breakers.
The most important thing is it makes it difficult to build the same tooling that other frameworks have. For example, in React you can either define your props as a TypeScript/Flow interfaces or use the `prop-types` library. This gives you an easy way to automatically validate props outside of the context of component instantiation. For example, you can do:
import MyComponent, {MyComponentProps} from "./MyComponent"
const props: MyComponentProps = {...}
import {propTypes} from "./MyComponent"
import {propTypes as otherPropTypes} from "./MyOtherComponent"
const MyNewComponent = () => {...}
MyNewComponent.propTypes = {...propTypes, ...otherPropTypes}
Also the component model isn't "hacky". It's logical, unopinionated and straight-forward. I don't know how much you know about Rich Harris but he's not one for producing "hacky" public APIs.
That's a pretty strange statement. TypeScript support does not mean you have to use TypeScript. It's purely additive by design.
By hacky I mean "define some global variables and the compiler will do some checks to make sure you don't do anything untoward". You can see my other post in this thread for examples where this model falls apart.