Reliable XMPP server, but there's a catch with paid broadcasts
I've been using diablo.team for several months now. Overall, everything works great: registration is fast, the connection is stable, and messages are delivered instantly. Modern OMEMO encryption is supported, so privacy is not an issue. The server operates without ads or tracking — that's a big plus.
However, there's one thing worth knowing upfront. The administration offers a paid feature: for an additional fee, you can send a promotional message to all users of the server in a separate chat created specifically for that purpose. In other words, if some service or project pays — everyone on the server will see that broadcast.
On the one hand, this is a way to monetize the server, and no one is forced to buy it. On the other hand, if you were expecting a completely isolated community without external notifications, this nuance might be unpleasant. I personally haven't encountered aggressive advertising yet, but the fact that this option exists is worth noting.
Otherwise, the server is solid. I recommend it — but with the caveat mentioned above.