Trying the Demo. Exported a 20k word text from Pages to Word. Section headings (three levels) and comments all appear in Word. Import to Manuscripts. Took almost 5 minutes. Sections not retained. Outline shows a single Section 1 with paragraphs below, no subsections.
Opening Section 1, by clicking on the disclosure triangle, requires 2 minutes to show the paragraphs.
Tried applying paragraph styles from inspector to individual paragraph to make them into section headings. Beach balled. No obvious way to change paragraph styles, at least not in the same way as the styles are applied in Pages.
So I'm discouraged. I was hoping the outlining view would be useful, since Apple removed it from Pages. I am hoping that the native file format is plain text or rich text and that formatting is similar to Markdown. But I wasn't motivated to explore further after I saw how slow the program responded on my Late 2009 iMac2.8 gHZ Core i7 w/8 gB of RAM, El Capitan 10.11.3.
Guess I'll wait to see how the application evolves. I have high hopes.