If you mean it as a user installed app, please disregard.

It completely falls apart and starts injecting nonsense phrases made up of nonsense misspelled words moment you miss a space ('v' or 'b') or type a really long word it doesn't know.

It also stubbornly incorrects other things like 'a' into "and" if it thinks it knows a phrase fragment. It's always wrong. It just happened to me now twice. Above, "or type a" became "or type and". Also, "it's" became "IRS".

Heliboard would be perfect if it stuck to word-only spellcheck and never split words. Just now again, it tried correcting "heliboard" into "hellenized" and "he lib oars".

This is not a matter of just lowering how aggressive the spellcheck is. It will still generate slop every time. The only real option is to completely turn it off. Futo can have similar problems, but there are a lot more options to tweak and the defaults aren't so bad.