With Famicom, you're more likely to encounter an exotic mapper when you don't suspect it.
I remember watching some of Pat the Punk's NES marathon, and someone requested Dragon Buster. One of the few games that refused to run, guess whoever made the charitable donation for that one to get played lost out. Turns out it doesn't work on PowerPak because it uses its own mapper. I can only imagine that's probably a low priority because it's only used by that one sort-of-obscure game. Seems Namco did several custom mappers, mostly for custom CHR swapping effects.