For a long time I’ve thought that the biggest shortcoming with MarginNote’s RTF/RTFD exports is that the documents produced are very poorly formatted. (The same can be said about the .docx exports.) There is a mishmash of fonts, point sizes, different styles of highlighting, etc. The reason to export notes is to make them immediately usable in other applications. (Not just DEVONthink, BTW.) Unfortunately, I have to reformat the RTF/RTFD/.docx files every time. This can be very tiresome for anything more than a few notes.
Also, the MindManager and OmniOutliner document formats produced by MarginNote are not the current formats followed by their respective developers. (MindJet 19 on Windows 10 usually rejects the .mmap files produced by MarginNote 3).
MarginNote – and now, especially, MarginNote 3 with its wonderful re-envisioning of what the app should be – is the best research environment on macOS and iOS. Getting data out of the app is just as important as the upgrades provided by the MarginNote team in the past year. Please consider making export a priority. Adding OPML export would be an excellent addition.