BookStack has existed and been constantly improved for 10 years, I think. It looks really great in the official screenshots, but the official demo is not filled out nearly enough to really give you a sense of the potential.
And with all that in mind, I think it's really strange that it's so hard to find a wiki or two using #BookStack, so that I can see how it looks and feels in real world settings. It has to be quite popular, right? So where are the wikis, how do I find them?
Here's a few pulled from our sharing channel in Discord:
https://wiki.opensourceisawesome.com/
https://books.metalrealm.one/
https://wiki.civlabs.org/
https://weraldiz.de/
https://www.anatomyarchive.org/
Many are quite customised, and these might not reflect the most common usages of BookStack (where it's used in business in corporate networks).