XMPP used to be the simplest chat protocol there was and basically worked the same on every client. However, today, especially with OMEMO encryption, there are lots of little issues. You might just have to fiddle with things to get it to work right. I use Pidgin on desktop and Conversations on mobile and I do have some issues but it mostly works alright. A lot of people have good luck with Dino on desktop as well. You’ll just have to figure it out :)
I don’t think it was ever the simplest chat protocol. Just look at weird things like resources, resource priority, MUC roles and affiliations, etc. You can still use it without E2EE encryption; all clients support that, but yeah OMEMO is becoming a “must have” thing.
Most people I know who use XMPP insist on using it with OMEMO so it’s become a necessary part of using the protocol.
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