nemo One con I can see is with xmpp you can't delete messages
I was not a member of the moderation team when it was decided. I was not involved in any discussions about it, so keep this in mind. I have also never used XMPP so I know virtually nothing about it.
However, if deleting messages isn't supported, that would be an instant deal breaker if you ask me. It's just not acceptable to leave certain things up for the whole community to see.
There are many people who've moved to Discord and many have said that they moved to Discord because it's a better experience overall.
Many people have issues with Discord because they don't want to provide their phone number. I personally signed up while connected to a VPN, I used a SimpleLogin email alias, and I haven't been asked for a phone number. Servers can set their "verification level" to "highest" (documentation), which would require that members provide their phone number, but our server doesn't have it set up that way.
Regardless, all of this isn't all that important considering all messages are still bridged across Discord, Matrix, Telegram, and IRC. Feel free to pick your favorite and participate from there.