Drab gray boxes may not be as 'engaging' or 'visually rich' but they are more accessible and usable. You know what's interactive, what components relate to what and how they affect the application.

The slack UI is a counter-example of a good UI. It's confusing, it's unpredictable, it's inaccessible. They made the application look like a fancy website instead of an application. That's a bad thing, not a good thing. There should be no attempt to replicate it. Using it as a benchmark to evaluate how flexible a UI framework is a non-starter.