What about extensibility? A very important reason why people use Confluence and Jira is there's an infinite supply of add-ons to do almost anything you may want to do, and then you can even implement your own.
Nothing user-friendly like confluence but some core systems are available to customize and extend. Of course, since BookStack is open source, some people have taken the option of maintaining a fork with deeper customizations.
https://marketplace.atlassian.com/addons/app/confluence