1) It has differences in behavior with certain classes and is not a drop-in replacement.
2) It always compiles, so it's kind of slow to compile-test
Compile/test time is ok. It's a few extra seconds to run tests, but hasn't been an issue in practice for me.
I've tend to have found Kotlin to be the direction I'm more happy going with. It speaks to my particular itches for me personally, more effectively. I can absolutely see how it's a very effective choice.
I wish the TypeScript/React integration was easier. Say what you will but there's no way you can achieve interactivity and convenience of React (et al) UIs with Turbo/Hotwire in a meaningful time.
Have you tried either Inertia (https://github.com/inertiajs/inertia-rails) or vite-ruby (https://vite-ruby.netlify.app/)? Both look very promising.
It's also got a bunch of semi-functional-programming paradigms throughout that make life quite a bit easier when you get used to using them.
Honestly, if it had types by default and across all / most of its packages easily (no. Sorbet + Rails is pain, or at least was last I tried), I'd probably recommend it over a lot of other languages.