I will produce multiregion famicom to nes adapters soon
Yeahhhh!!!! That's good news. Would you also build the other converter (NES to Famicom, 72 to 60)?
I have a Famicom and to play NES games the converter I bought had to be modded (Castlevania 3 fix) and just for completeness I had to add the 10K resistors split circuit on the audio path (you never know when some new homebrew project would start pumping audio there).
The main problem that I found is that the top connector on the converter (the 72 pin, where you put the NES cartridge) is not exactly to spec, it is very similar but not perfect so that when I insert a game I need to make sure to push it all the way to one side (can't remember left or right). In Castlevania failing to do so will render garbled grafix so I know what to do from the get go rather than half way thru the game. I can see that the issue is the connector connecting 2 nearby traces if not aligned correctly, thus shorting a couple of signals, cheap hong kong converter, maybe it is just very hard to find the exact 72 pin connector these days and they just used the closest they had.
If you would make this type of converters I am pretty sure you would not overlook the tolerances and it will fit perfectly so finally we could play Nes over Famicom without so many issues.