The main problem with XMPP (well for me anyway) is that the implementation of OMEMO that 99% of the apps use is woefully out of date, and Conversations (and their ilk) seem to actively resist updating because unless EVERYONE does it at the same time it will break interoperability.
The second problem for me is the ease of use, I'd have to physically set up accts for literally all my contacts on Jabber (everyone I know is stupid [it's really just learned helplessness and it pisses me off to no end]), but on Delta I can legit say "here, click link" and it'll bring up their app store on DC, then they DL and make an acct and then I say "click the link again" and they're directly connected to me, done. So much easier. And then next time I see them I can say "hey give me your phone I'm gonna do stuff to your Delta acct" and I can add relays and create a backup for them, but at least we can get started easier.
Besides, they are working on forward secrecy afaik, and they're one of the only teams making these apps that seems to acknowledge metadata's existence, honestly they seem like one of the best options all things considered. Sure if your threat model is the US gov then maybe not, but better not use Intel or AMD or any android (incl Graphene, and I'm a user myself) or iphone then. Carrier pigeons maybe.