ActivityPub and Mastodon brought new incentives into the world of decentralised communication platforms, even so far as I would call it a serious alternative to platforms like Twitter. But all efforts made by hundreds of individuals every day – administrating servers, developing software and moderating communities – have a weak spot which needs to be addressed in the near future: who has control over the underlying computing infrastructure of the Fediverse? And are users aware of the conditions?
Not all rooms are private. When joining a public room, many users assume that only Matrix users can see the content, similarly to IRC. Instead, it’s equivalent to writing on a public website.
Plus, people make mistakes, e.g. typing in the wrong room. The built-in edit mechanism is unable to handle this issue because everything is made public immediately. Deleting a message can even give people a false sense of security.
These are not problems of Matrix but of the UI/UX of the clients you are using!