Hi! As far as the hardware goes, I've written down some of the history on the listing itself.
Beyond that, afaik Mr. Hatanpää (the hardware & firmware developer) gave up on the project around late 2006. As I recall it from the top of my head, the reasons for this were that he didn't have time for the project and also the CPLD chips were becoming difficult/expensive to source.. But I'm guessing the release of PowerPak cartridge (around Q3 / 2006 iirc) may have served as a catalyst too; including SD card with the cart just makes much more sense and adding this to FunkyFlashCart would've required a lot of efforts to re-design & re-develop.
As for a bit of personal history with the cart: I was pretty active with the Demoscene all 2000s and up to around 2015 writing music to a whole lot of productions (see list here). My interest with the cart was squarely for this use - to have a go at writing music for NES. Back in mid 2000s, some of the Demoscene events that held competitions like executable music for retro computers and consoles, required you to prove that what you submit to the competition actually works and runs real-time on piece of real hardware. This was mostly to capture audio & video from it, to then be able to screen the competition without running into possible hardware issues. Luckily emulators and other tools became way better even over the 2000s and so this competition requirement disappeared for common retro platforms
Anyway, halting the development is also why mapper support is incomplete for this cartridge. Even if Hatanpää made source code freely available, it made little sense for anyone to pick it up as-is for the lack of SD card slot, chip sourcing issues etc. Still, I think for any of these carts that might be still around (and esp. in working condition), they are one interesting curiosity in the history of NES homebrew & DIY.