Most people don't care about any shooters pre-16-bit, understandably. I mean, as far as home consoles go. But this is a really charming game. I've never played Recca, btw.
I own Gun-Nac on Famicom but haven't done the "deep dive" into it yet. The invading rabbits in the first level definitely gives it a unique feel right from the start. And Compile makes an 8 bit CPU sing like no other developer out there.