That’s true, and Craft is a fully-featured and visually appealing native app. However, I add hashtags to the headings and subheadings in OO and then export the outline as plain text, which results in a markdown outline. There are also dedicated markdown outlining apps available. I used Craft for a while, but I don’t enjoy writing in blocks and I don’t require most of Craft’s features. But, as I said, it is a beautifully, powerful app. Frankly, I wish I could find a good use case for it. 
Outliners have to fit aesthetics as well as functionality, and it’s interesting how much perception means.
For instance, almost the same sequence of keystrokes in Bike, OmniOutliner, and Mellel (with outliner tweaks) will produce identical outlines.
Yet, some writers find OO too fussy and outlining in a word processor sounds dreadful - but it works with pretty much the same keystrokes you’d use in Bike.
The outliner tweaks I use in Mellel are a style set dedicated to outlining which includes the Auto-Indent feature enabled.
Mellel ends up being very capable as a two-pane outliner. Generally I reach for OO over Mellel for outlining, but it’s nice to be able to add sidebars and document comments for notes-to-self. Having identical citations in the outline and the manuscript might be nice for anything requiring a bibliography.
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