Fable is great but it has a surprising number of these hidden behaviour changes that are really hard to detect when writing code against it.

I haven't used Fable much, but apparently it maps .NET arrays to js TypedArray. Presumably you could keep the registers in 8-element array and fable will properly produce a Uint8Array. I'd like to benchmark that.

It’s really hard to please everyone all of the time on this front.

This kind of thing is why Roc compiles to WASM but not JS.