The BK-7m and FR-8x were made for each other. The setup is simple, the configuration made for the 8X (it's a simple wizard and configuration save so that it remains the same with every reboot).
Pros are a nice set of rhythms, cons are that there are not enough rhythms for all needs.

Cons, its not made anymore and they do have a known issue with the selection wheel if you are not gentle with it. A pro is you can control the BK-7m with an app. Another pro is that the 8X can control the BK using it's chin switches.
Biggest Pro at least for me... the endless sound combinations and of course you can control the presets/registrations all wirelessly with the addition of wireless MIDI and an iPad.
Pros and cons are going to be more varied based on the needs of each person. I likely use it in near it's most basic modes where others expand their sound choices/options by using the tones available in the BK-7m.
If it doesn't come with one, you will need a foot pedal. I made my own:
Time and patience are key. Downloading and reading the free PDF from Roland is never a bad idea.
I found a zip file online some years back that contains like 2000 extra rhythms for the BK-7m. Loaded it once to see if it worked, it did and then I went back to using what was built-in.
If you've had the BK for a while, you should have a good idea of what it can and cannot do. It does exactly the same on the 8X, and adding an 8X doesn't magically add any extra sounds that were not there before with your MIDI accordion, but it does add a bit more flexibility because Roland made both and designed them with some compatibility for each other.