* They're not portable. My username here is Zak. It is on reddit as well, but I think that's the only other place. If I don't discover a service within it's first week of operation, I won't get that username, and many won't allow it because it's "too short".
* They don't tap into users' existing contact lists. I've discovered several people I know using Signal because I had their phone numbers stored in my phone's contacts.
But if a new communications protocol is to replace the legacy phone network, which I consider desirable, it probably shouldn't use identifiers tied to the legacy phone network.