Last time I installed slack through flatpack I couldn’t send any files. Not through drag-and-drop, neither through the filepicker. The latter was just empty.
Downloading files from slack also had awfully weird side-effects.
Slack doesn’t have an apt repo, so I download debs and updat manually. Maybe once half-a-year.
If that’s the experience I’d get on my signal through flatpack, I’d also rather be downloading manually. And I’d even compile from source rather than deal with that flatpack stuff.