As Jerry said, the default styles in the BK-7m are primarily focused on modern music, and quite limited if your interest is folk music. But there comes one extremely powerful feature. Using any midi sequencer software and a converter software freely available from Roland, one can create custom styles and save them into a pen drive for using them in the Bk-7m.
I still did not have much time to play with this; it is not a straightforward process and requires time, but in a couple afternoons I managed to create my own -very simplified- Argentinean Zamba and Chamamé styles.
As for using the module as an expander, there are a few tweaks to be done in the V-accordion settings (at least in the fr-3xb and fr-1b that I have), and also in the intrincated menus of the Bk-7m, in order to have the module using bellows expression instead of key velocity as it does by default. (I read that this may be a problem with the new fr-4x, but cannot speak for that). As you say there are a ton of different sounds inside it, although I am missing non-modern percussion banks in that area.
I'll also add that by using an optional seven switch pedal one can trigger functions on the fly as you play... start stop rhythm, adding automated second voices (although I am not satisfied with the way this has been implemented by Roland, this feature in my humble Casio keyboard is far better), and a lot more.
Edit: I forgot to add that building your own pedals is simple as long as you have some soldering ability. I made my own for a total cost of some 30 euros.
I've had the module for half a year now and only used a fraction of its functions, need longer days to experiment! I got mine for 450 euros second hand, and in fact it was like new, only lacking the owners manual.
A great and powerful addition for a midi accordion, especially a Roland one.
