They are "great" from the perspective that the non-tech-savvy user somehow can get into using these. This did not work for XMPP (they need to learn that they need to pick a provider and a client; then they need to deal with entering the credentials into the client; then they need to solve problems like history synchronization and access from multiple devices; and finally voice/video/filesharing over XMPP is a disaster).
I agree onboarding can be a hurdle (for any decentralized network). These days many XMPP clients support invitations which make the process much easier (see https://blog.prosody.im/great-invitations/ ).
This easy onboarding is a fundamental part of Snikket - allowing you to bring groups of people onto XMPP by just sharing a link with them.