Others are noting clients & servers matter. This isn’t a downside—it’s just that the protocol is flexible & extensible for many types of messaging beyond human2human private conversations, which explains why encryption isn’t a requirement for the clients. With that said any modern client targeting said H2H interaction will have basic forms of encryption like PGP, OTR, & OMEMO which all do the job of E2EE. OMEMO is based on the same ideas that Signal, WhatsApp, Matrix, & so on use so that part is all the same.

A unique feature for XMPP in this space tho is how low-spec & resource-unintensive the servers/clients are—you aren’t chewing up a ton of CPU or RAM, there is no eventual consistency to balloon storage (MAM is enough), clients don’t drain your battery or take literal minutes to sync with servers. Since it is low-cost, it is feasible to self-host XMPP from a residential server (at home on some old hardware for instance) or add it to a multipurpose machine where it doesn’t get in the way of other processes/storage. Some of the other service often mentioned here either you can’t self-host or are quite expensive to run (often by design) which limits the accessibility causing centralization as well as requiring trust in that server you don’t own.

E2ee is not everything, as most of the privacy sensitive metadata can still be collected. Sure it is nice to have, but even more important is that you can chose a trustworthy server operator or run your own. XMPP allows doing that, but it has some weaknesses with client implementations and so on.

I am a bit biased and would say all in all XMPP is probably the best option right now, but it depends on your specific priorities. It certainly has some rough edges though.

You have to trust the servers with your metadata, and that the servers have their inter-server communication locked down, but at least you can choose/operate servers.

Some clients are a bit flaky with their e2e encryption defaults or from a UI perspective it is easy to send an unencrypted message (in a new chat for example) before noticing that was how it was set.

There are a few XEPs the server needs which enable things like OMEMO, efficient mobile data/battery use, offline and multiple device deliverability, file transfers, etc. Audio/video calling has various requirements as I think xmpp only facilitates the setup of the call.

its really really save

Edit: sorry I misread it as XMR.

XMPP can be very unsafe. It depends on the client you use. Its best to use a protocol that doesn't allow unencrypted messages to be sent at all. Like Wire or Signal.

It depends on the client and the security implementations they support. For example IIRC no client support the last version of OMEMO (I think it was about OMEMO, I remember an article about it some time ago). Also are you sure that all the other people's clients are on the same version and you're not susceptible to a downgrade attack?

Unless you are ready to/want to control the whole environment (i.e. at least the clients and possibly the server), look into simplex.chat