Separate clients for Linux, Windows, Mac, Android and iOS. And last but not least the web client had to be a good one.
SFOS support would have been a plus, of course, but with only so few users with SFOS, hardly a deciding factor.

After running XMPP for 5+ years we tested Hip.chat (RIP), Mattermost, Matrix and Rocket.Chat to fight off the ever invading Yammer and Flowdock solutions. Those weren't allowed but people used them anyway.

XMPP situation had been bad for years; no viable web client and only development forks that were killed left and right. I ended up forking Jappix (RIP) for our own use, but that didn't scale in the end.
Also the best single XMPP client, Conversations on Android, was not ported to traditional OSes.
Pidgin and Gajim just don't cut it. Pidgin is an ugly swiss army knife tool for IT-advanced people, and Gajim is not any better from the users point of view.

Hip.chat was clear for me that it was in the end of the road, and indeed Atlassian killed it soon after. It wasn't open source anyway and was expensive.

Mattermost was promising but their clients were afwul, and their support for full names and online status bad. Also they were charging small diamonds for the version with advanced authentication solutions. (And source code built it as a docker image.)

Matrix has the same good vibe to it as XMPP has, server-to-server communications and all, but the web client riot.im was years away from the last tested contender, Rocket.Chat. The web client was ugly as hell (themes too) and had lots of functionality missing.

Rocket.Chat has its bad sides to it (noSQL database mongodb and no s-2-s communications), but its web client is excellent and the clients for all mentioned platforms are good, although the mobile versions keep evolving constantly, and not always without regressions.

Edit: This happened two years ago, and I haven't checked the situation with Matrix now. But at the time they seemed to have their priorities off just like XMPP scene had had. In fact it was worse; regardless what they said in their FAQ about not trying to compete with XMPP, they were fracturing the XMPP scene just when Slack and Flowdock took over the world.
I give credit for XMPP and Matrix for what they are trying to achieve but meanwhile we are stuck in our own shard of Rocket.Chat.