Some of them are just bizarre. We have like 14 different WordPress sites with varying degrees of customization. Oddly, some of our most basic WordPress sites (basically zero plugins) would return _no results_ when querying for Posts. Especially in our Multisite instances. Especially in our sites that hook parse_request. Some others would return inconsistent results for the same queries. Custom post types would most-consistently not return sensible results. Entries in the wp_postmeta table stored via ACF Pro seemed essentially inaccessible.
Most annoying though is that the API returns paginated posts but without returning data about how many posts there are or what page you're on. IIRC, there was a hard limit on returning max 100 posts per query. Might have been higher but significantly less than the number of posts we have. It was a total nonstarter unless we could guarantee that we could make and return enough queries to return all the posts before anything changed. With a 40+ content team that's impossible.
Sorry that I can't be more accurate than that right now.
Side note: I don't know if this has been said before, but ACF Pro is like the single most essential WordPress plugin for dev teams who know what they're doing. That plugin combined with some dev time replaces the need for like 99% of anything else that we would need to install. Please consider doing whatever it takes to make that plugin part of WP Core. Installing it on any site is a really sensible thing to do. That said, their field labels ("field_") really should be replaced with a GUID.