Now that I look at it more, it seems to me that these concerns may be mostly about “theme” aspects. The first thing that stood out to me in Birch was the small disclosure triangles on every line. One of the things that stood out to me about FT was that there weren’t any of these traditional outliner interface elements present. One the one hand, this leads to some of the conceptual confusions Rob mentioned in his post (how to determine outline depth: indents or headings?). On the other hand, it led me to feel free to just write in FT as a normal text editor, but then gave me the ability to add in some light formatting and take advantage of folding, filtering, etc. It never seemed like an outliner to me, but a text editor plus.
I am fully behind many of the affordances Atom provides (tree view, tabs etc.), so perhaps once the theme is set, it will actually end up as “free form” as FT felt, with the added affordances just mentioned.