Slack is browser based, even the "native" client option. This means your colleague cannot choose which screen to share in Linux, so if he has two screens he has to go at great lengths to make it work properly (either share both or share a specific app instead of a screen).

Also Slack's screen sharing quality is terrible and ofter you cannot read letters in the remote end's screen. Zoom's codec is much more crisp with text and graphics, almost like a native app running in your computer instead of video.

The Slack screen sharing is so laggy it's painful. Which is weird because pre-acquisition Screenhero was incredible. It was the best screen sharing software I'd ever used.