@JustinThought - I agree in principle, but:
IIRC Word's Grammar and Style checks have always been optional (none, grammar only, or grammar & style) and configurable e.g. don't want to be told you've split an infinitive, there's a check box for that
Besides which, all the messages are advisory, grammar and style messages don't have a Change All option, but they do have an 'Ignore Rule' option, although the latter may have been added since the current spell checker first appeared in Word '95.
If you have a clean conversion, i.e. one not riddled with OCR scanning errors then, particularly for fiction, why would you even check the spelling, let alone check the grammar and style. As you alluded, regional, and historical cant etc are often essential to the story, especially its humour.
Non-fiction is a bit different, US English grammar can be sometimes be awkward or ambiguous to a non-US English reader, and vice versa of course. Example, because it grates on my sensitive UK English ears, for non-fiction I use a Word exclusion list that includes the word 'gotten'
But I decide what to do about them on a case-by-case basis.
Toxaris' ePub-Tools add in for Word, features a spell checker that uses a similar user interface style (i.e. a list of words) to that used by the Sigil and Calibre ePub editors' spell checkers. IMO it is a better interface than Word's word-by-word approach - most of the time.
BR
Just for fun, this message was checked by Word (spelling, grammar and style), ePub-Tools (spelling), and LanguageTools (grammar). Nevertheless, some errors were not detected; nothing beats reading in a different page size, font etc.