The matrix homeserver is... suboptimal. I'm running it for a personal instance (two users), and it's using insane amounts of RAM (4-8 GB) and still managing to lag (even though I gave it 4 cores on a Ryzen 2700X). Forget talking in a matrix.org room, even talking to people on other personal/single user servers isn't fully stable.
Also, client support is awful. The only one I can reasonably tell a non-technical user to use is Riot - but that's somehow more resource hungry than the Slack desktop app. There are good ones, but they are either only for technical users (weechat) or are extremely barebones (nheko, quaternion).
There is a replacement homeserver (https://github.com/matrix-org/dendrite) but it's not done yet.
Also, client support is awful. The only one I can reasonably tell a non-technical user to use is Riot - but that's somehow more resource hungry than the Slack desktop app. There are good ones, but they are either only for technical users (weechat) or are extremely barebones (nheko, quaternion).