There are some hilariously ugly things for even trivial things like editing/replies:

Edits get a `"m.relates_to": {"event_id": ..., "rel_type": "m.replace"}` field in the body, and `"m.new_content": {...}`, containing the plain-text and HTML versions of the message (while also keeping a copy of both, typically with a prepended "*", outside "m.new_content", for backwards compatibility; yes, that's 4 (four) copies of the message text in an edit; for some while Element generated up to 10 (ten) copies IIRC, due to some proposed extension(s), but that seems to be gone thankfully).

Reply messages get `"m.relates_to": {"m.in_reply_to": {"event_id": ...}}` - an annoyingly different format from the edits. It might look like that that allows an edit to change which message is being replied to, but nope, last I checked, that's not supported. Oh and for backwards compatibility a <mx-reply> HTML element is to be prepended, containing a copy of the replied-to message and its info (and yes, that means that the replied-to message effectively can't be deleted as the reply will still contain its text; luckily clients can choose to do not generate such <mx-reply>, but Element still does). And if you want to actually get proper reliable info about the replied-to event, you have to just make an API request for every single one you want to display (unless you happen to already have a cached copy, which luckily for replies is reasonably common).

And then there's threads - again in the name of crappy backwards-compatibility there's a mess - within-thread replies imitate a reply to the last in-thread message, along with `"m.relates_to": {"event_id": ..., "rel_type": "m.thread", "is_falling_back": true, ...}`, that "is_falling_back" indicating that this isn't actually a reply (being false when you want an actual reply). And clients are "supposed" to also handle replies to in-thread messages without the "m.thread" relation (which'd come from clients not supporting threads), but as far as I can tell there's no way do to it while paginating without making an API request per every single message (and yes Element behaves quite buggily here).

And then there are some things that can't be reasonably handled - the context/message listing/pagination APIs don't give any reaction info (besides the reaction events themselves in chronological position of their addition), so reaction presence/counts in history view must be calculated by clients, and thus won't be able to show ones that were posted a while after the messages. (there used to be some aggregation provided, but it's since been removed!!!) I think the only way to do this properly is truly just making an API request of "list reactions" for every single message the client wants to show.

This may make it seem like Matrix has an extremely firm stance on backwards-compatibility, but nope - recently they deprecated & made non-functional the unauthenticated API retrieval of media, making it impossible for clients not supporting video/audio/image display to just open those in the browser, instead forcing them to have custom file downloading & saving code, and also making it impossible to link to media within a message. There was a window of 6 (or less?) months between the new API being finalized, and the old one being removed.