@mrcurrieSpaces before and after an em-dash are a Style Guide issue (i.e. it's allowed for specific publishers, and not for other publishers), yet for most SOL authors (technically 'self-published' authors, it's best to have your own standards, so your writing is more consistent and not chaotic and random.
Thus, I've always dropped the extra spaces, yet that too pisses certain readers off. Yet, in those specific cases, I just shrug and ignore the complaints, as it's more common in non-fiction than it is in fictional uses (formal vs. informal writing).
Of course, em-dashes, at least in fiction, can also indicate an interrupted sentence (just as an ellipsis can also indicate a 'hanging sentence' where the speaker intentionally doesn't complete the sentence, as letting the listening make the necessary connection is more impactful) in which case, if the same speaker continues, it's considered a separate sentence, requiring an intervening space and a capital letter for the next sentence.
English can indeed be a very weird language at times.