One of the disincentives for people trying Backdrop is the worry that they'll spend lots of time and money setting up a system based on it, and then later finding themselves prevented from using a different system because their data is locked away in a proprietary format. We should ease these fears, by making it easy for data to get in and out.
To import data, a variation on Feeds module would do the trick. It wouldn't need to have all the customizability of feeds -- it doesn't need to poll, doesn't have to support multiple file formats, doesn't need to be infinitely configurable. It could be built into the content type definition screen.
To export data, it would be enough to simply create a CSV file for each content type, term, user, and comment type, as well as the necessary CMI information to create it.
Wordpress supplies this kind of capability, to a degree, in its generic interface:
@kwikstrt, did you try this one? Did it work for you?