Matrix clients do not allow to even select several messages to delete them at once. Matrix is a superior concept to proprietary Signal, Whatsapp, Telegram etc but the implementation needs some help yet and UI is far behind.

No it doesn't. Matrix has large groups, but they are limited:

1. You must join a group to see its contents. This puts a stupid "{user} joined the room" message in the room for everyone to see. Channel membership is anonymous.

2. Channels generate a public feed that doesn't require any special software to read from (see Durov: https://t.me/s/durov). Matrix requires a client of some kind, all clients are clunky at best and none have a public view.

3. Channels support millions of readers. Matrix rooms literally cannot do this, and rooms frequently get out of sync due to federation.