I didn’t mention a lot of things, for various reasons.

Like GP said, once Google killed XMPP compatibility, it lost a lot of luster, but I was using it long before Google Chat added integration and I mainly remember fighting with it a lot. Maybe it’s improved through extensions, but when I left it:

• Group chat was weak. It exists, but I’ve never encountered large, persistent group chats in the wild.
• There was no server side message caching; if you weren’t online, you couldn’t get messages
• There was no message syncing. If you used multiple devices, you received messages only on the active one(s?).
• Feature support varied wildly between servers, and you were never sure what you could use with any given contact
• Encryption support was spotty and an after-thought.

A lot of this isn’t relevant to group chats, but the lack of history syncing was a big one, and is also one of my main objections to IRC.

Speaking of IRC, it may be possible that with the right combination of clients and servers you can address these, but IM recent E with IRC it lacks some QOL features, some of which are, for me, minimum requirements.

• The history thing. If I go offline and come back, all conversation since I last logged out is unavailable.
• It lacks basic support for message editing. There’s the s// convention, but it’s handled client side (or by a bot), and it’s only a convention and not a spec
• There’s just a lot of creature comfort stuff that’s certainly not absolutely necessary, but which makes modern IRC less pleasant to use, and for some teams, deal breakers. Channel level file attachments. Inline images. /react, /reply … just a bunch of convenience things that are either only supported by some clients and so discouraged, or just not there.

But the main thing is that it’s not federated. It may look like it is, but that’s only IRC clients letting you connect to multiple servers at the same time. You have to have accounts on each server - there’s no ID sharing. If there’s any channel message syncing, it’s done through bots, and requires collaboration on both servers. OP specifically asked for federated solutions, and IRC isn’t.