XMPP has issues on Android and iOS: the app needs to be running and connected to the server at all times to fetch new messages. iOS explicitly disallows this from what I understand, and on Android you are at the mercy of your OEM's battery optimizations unless you specifically keep the app running by pinning it or if the app has persistent notifications. Either way, not something that is easy to explain to normal users, meaning pain points arise very soon.
Matrix is a little more promising, but Element (at least on matrix.org servers) is slow, especially at scale.
Any chat app would have issues with battery optimizations. Solution is usually to use the mobile OS vendors push notification infrastructure. Work on that in XMPP started around 2015 and most popular clients and servers should have some support by now. It's tricky to make something nice while the platform is working against you tho, especially while those push notification systems use/d XMPP themselves.