One way it explicitly doesn't scale (unless something has changed since I last checked and my quick search failed me) is the hard limit on 8000 bytes of data in a notification. If your notification doesn't make sense to exist as a row (where you can just give an ID), then that makes it hard to use. I had a web game and the events were transient descriptions of changes in state that didn't make sense to store in the database, and could be bigger than that, so it didn't work for that use case.

As an implementer of a scalable notification system I'd be sure to cap the notification size.

- Keeps messages O(1) so I can focus on scaling in the amount of notifications

- Tells whoever runs into this that,

    - I didn't planned for arbitrarily large messages as they *might* otherwise grind performance to a halt.
    - They *might* be misusing my notification system