If one mistypes a word then it is underlined with a series of red dots (just as it would be in (Mac) Scrivener). However, when the word is corrected then the red dots persist even though the word is now correctly spelled. It’s a minor point but it is intrusive for those with visual perception issues.
Unfortunately that is an Apple bug, you’ll find it in other programs as well. For example if I type “doesn’t” into ByWord, the ‘doesn’ part will remain underlined in red, because the spell check engine checks at punctuation, thus evaluating ‘doesn’ all by itself, and doesn’t update the display once you finish keying in the contraction (which will later be detected as one word and not underlined).
Okay, if it’s an Apple bug at least it’s known. I don’t remember seeing this happen in other apps (e.g. Textilus or Notability) that were my stop gaps for Scrivener for iOS but then the stop gap apps had real problems of their own. So glad to be shot of them.
Not sure about Notability, but Textilus was using a modified WebKit view instead of the standard text editing view (back in the day that was the easiest way to get rich text). I don’t recall it even supporting spell check at all—but maybe they eventually fixed that. Whatever the case, it wasn’t a standard setup, and as you note it had its drawbacks as well. We looked into that approach years ago, and it was pretty awful—as you can imagine trying to make a rich text editor in what amounts to a web browser engine would be.