I built https://www.listenaddict.com/ with Sapper. It was a good experience. That said, all of my other apps are using Kit, and I'm finding that experience even nicer.
This post [0] sums up my general feeling toward SvelteKit. It seemed very opinionated and very restrictive, two qualities which seemed opposite the philosophy of core Svelte.
Yes, it's very much in beta! And if that's a concern for you, then I'd stick with Sapper or Elder.js or another.
That said, I think the replies covered the concern pretty well. For me, I don't really have any concerns with it. I don't find it very opinionated nor restrictive, but I come from a Rails background where I'm used to convention over configuration, so maybe I'm biased towards reasonable defaults.
[0]: https://www.reddit.com/r/sveltejs/comments/ni9g5d/svelte_vs_...
Do you have concerns with using SvelteKit in production? It's still in Beta last I checked.