It's an entirely closed source, proprietary codebase, run by a for-profit company where you have little control over anything. These corporations don't care about actual users and they will leave you high and dry. There is a reason people still use IRC - it's open, easy to connect to and has been around for literal decades. Remember CompuServe? AOL? AIM? ICQ? Google Chat shutting it's doors to xmpp? If so, you understand the pattern. It's about walled gardens and blocking interoperability. The industry doesn't need more of that. We are chatting on an open source link aggregation site because bean counters at Reddit decided to shut off APIs to existing apps arbitrarily.

The matrix stack solves most of those problems by providing an open source codebase and protocol, easy to connect to solution that is akin to Slack. I am fortunate enough to not have to use discord much beyond checking on a class schedule and downloading some sheet music, so I will never be a discord power user. Maybe some there is crazy awesome feature that discord provides that no open source platform does, but I have some serious doubts about that.