Matrix would be a much better choice IMO.
It's worse in every way. It's not even close to something as polished as Discord.
1. "Open in browser" is a good start, but then the registration process is downright painful. Discord doesn't require you to register at all to try it out, just to pick a nickname, it's vastly better for onboarding.
2. Requires unique nicknames. A chat app with unique nicks? this has to be a joke right?
3. As usual with FOSS, the worst part is the UI/UX. The whole design is extremely confusing. At first glance it appears the app only supports 'rooms'. But it seems that there are 'communities' and 'spaces' too? I have no idea what's going on. Discord just has servers with channels and private messages and it's all clearly indicated in the UI.
Listen, I do want to use open and free alternatives if possible. But let's be realistic and stop pretending that something like Matrix is anywhere near Discord. A more reasonable alternative would be Zulip.