It has become among the most popular open messaging protocols, but it remains a scalability nightmare.
It's literally the Blockchain of messaging. Its current state is given by the sum of the whole chain of events received since t=0. It's a brittle append-only ledger, and any modifications you do to it are very likely to break it.
Do you have a huge 100GB database and you want to clean up old stuff? Sorry, you can't. There are some non-official solutions for compacting the events, but they're all likely to break your db - and none worked so far in my case.
Do you have users on your instance that entered a busy room on matrix.org? Then you'll get all the join/leave events of that busy room on your db, with no way of deleting them, and nothing to do unless your users exit those rooms.
Did you start your instance by toying with the default SQLite backend, and now that it's become big you want to move to Postgres? Sorry, no official guides provided, only unofficial procedures scattered across a bunch of blogs.
Do you want to change the name of your server? Sorry, you can't. All the events are tightly coupled to your server name. The advised solution is to simply start a new instance.
Such a rigid and brittle implementation shouldn't have become an open de facto standard without much questioning about its poor design decisions.
At the very least, some official tools must be provided to enable admins to compact events. If the size of the database is guaranteed to increase indefinitely, then entry barriers against self-hosting are only going to increase.
