Signal is a non starter. Never had an account and barring major architecture changes, I never will. Weakly secured centralized control of metadata with no option to self host your way out of it, and a requirement of a phone number which makes it a non starter for friends in countries that have good reason to be anonymous.
Zulip is at least self hostable but all private conversations in plain text on the server and lock-in to only being able to access chats on the one server are also both non starters. It is not a matter of trusting the admin, it is trusting that the Linux server was deployed with no exploitable flaws, and the admin never gets a court order. Too much trust for me.
Zulip is maybe a nicer UX alternative to IRC, but it is not comparable to Matrix.
Zulip's topic focus chatting get the work done and kept information organized, very easy to search and assign tasks.
That's huge upgrade over IRC since information is organized at the protocol level.
i agree matrix is desirable to replace slack only in case of where E2E is necessary.
For companies, inspection of chat logs may needed by top management in case of dramas.
Zulip is at least self hostable but all private conversations in plain text on the server and lock-in to only being able to access chats on the one server are also both non starters. It is not a matter of trusting the admin, it is trusting that the Linux server was deployed with no exploitable flaws, and the admin never gets a court order. Too much trust for me.
Zulip is maybe a nicer UX alternative to IRC, but it is not comparable to Matrix.