I'm biased, as I lead the Zulip project. But I think this is a reasonable place for me to post some thoughts. Given current events in the USA, I can't emphasize enough how worried one should be about the fact that a few companies like Discord, Google (Gmail), and Meta have databases with access to the private conversations of hundreds of millions of people with their closest friends and family members, linked up with…
- Your server admin can see DMs (or at least metadata, not sure if Zulip does E2E for DMs). The same is true for centralized services in theory, but unless you're a terrorist or a person of interest to a major government, it's extremely unlikely that a Discord employee will have an incentive to spy on your messages specifically. Your admin is likely part of your community and may know you personally, so the temptation is much, much higher.
- If the admin dies and nobody else has the keys to the kingdom, the server can go down at any point, and there's no way for users to reconstitute the network semi-automatically. Discord servers don't just go away unless somebody actively makes them to.
- It's much less secure in practice, it relies on your admin to always be on guard and constantly update their server to prevent vulnerabilities, either in Zulip or in the myriad of other self-hosted services running on it. One guy in his basement that goes on vacation once a year and has family responsibilities is far more likely to make mistakes than a team of trained cybersecurity professionals.
- Many Discord users are in 20+ servers. Anything that doesn't provide a one-click server joining experience (for users who already have an account on a different server) is nowhere near a Discord replacement.
- People want bots (for things like high-fidelity Youtube music streaming on voice channels), and those are mostly Discord-only.
- Anything open source will be worse at phishing and fraud / abuse prevention by definition, as many fraud-prevention approaches rely on the fraudster blindly guessing at what the code and ML models (do you even have ML models for this) are doing.