It's not accessible. This alone destroys "beautifully".
But even beyond that, it's just not a good citizen or experience wherever it is. It's deeply single-paned and hence single-tasked: you can't open a conversation in a tab or separate window. And switching between conversations/threads/groups on any platform is much harder than it needs to be. Their quick-switcher is a quasi CLI bandage over this that increases cognitive load on the user.
The enterprise multi-slack experience is even more horrible as it expands that problem across multiple quasi-discrete instances.
On iOS it does unnatural things with text so you can't select portions to copy and paste.
On desktop it is so much of a resource hog it's our generation's version of Eight Megabytes And Constantly Swapping.
And this isn't intrinsic: none of this stuff was an issue when you could use IRC clients to access it. But they turned those off.