That’s a bad implementation then. Modern open-source XMPP works great on mobile
The issue was the state of mobile clients when XMPP died in the mainstream and state of the art was crap like Xabber. Conversations was better but too little, too late.
iOS is more of a mixed bag, but that is solely Apple’s fault and applies to all messengers other than iMessage.
Telegram works flawlessly pretty much everywhere, including iOS which my mom uses.