There isn't a single XMPP client for iOS that properly follows the iOS HIG and isn't ugly or unusable. There are very few clients that are even fully featured. The XMPP landscape on macOS is even more bleak.
Isn’t this true of most open source software in iOS?
The intersection of developers who want to develop on and for a proprietary walled platform and also want to work on open source clients for open standard, descentralised protocol must be pretty small.
As an open-source XMPP project, we tried hard to find iOS developers willing to help with development (as volunteers, or even paid) and it's just so hard to find people in this intersection of interests.
If anyone reading this thread has iOS dev skills and cares about improving open-source messaging on iOS, feel free to get in touch.