Their business model is fine for many groups of user. However - open source projects in particular should be thinking about archiving, accessibility, discoverability etc. Slack is a bizarre choice which I can only ascribe to "nice UI and good timing".
I'm personally getting a bit sick of it for another pragmatic reason. It's bloody slow to open, slow to switch accounts and even slow to switch channels.
There is very little "nice" about their UI. UX isn't good (subjective, I know); it's not accessible at all; their "apps" take gigs of RAM and waste CPU, very slow search, not intuitive, etc.
I was able to search years of intensive mailing in both server and local cache in an almost instantaneous fashion in Outlook {2003, 2010 & 2016} for Windows, but Slack can't properly search a year and a half of history without choking. On almost every level, slack is a complete technological failure (where it matters).
I'm really not sure how the two can be separate. In user interface concepts, UI design and UX go hand in hand. As a matter of fact, they do not just for UI but almost everywhere else. Case in point, Apple's glass windows that caused people to hit their heads in them by mistake. Terrible design and terrible UX.
I'm personally getting a bit sick of it for another pragmatic reason. It's bloody slow to open, slow to switch accounts and even slow to switch channels.