I did a comparison between the two versions (4 and 5) using a trio of epub2 files that had errors when purchased and I saw no real differences other than some error messages only appearing once with the beta. One of files was a puzzle since it was downloaded from Google and says it's an epub2 but quite a few epub3 specific bits and bobs (epub:type, role, etc.) are in it which triggered errors. I keep that original of that one around just for testing.
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