me_irl

They are not. They're architecturally very, very similar but there are some small differences between the Japanese and US machines.

Some of this was very deliberate on Nintendo's part. The pin count on the Famicom cartridges vs. NES ones is different, and the Famicom lacks the infamous 10NES lockout chip which was present in the US models and their cartridges. Famicom game piracy was rampant in Asia prior to the US NES release, and Nintendo didn't want that to play out again in the West. The NES was made incompatible out of the box with Famicom carts not just to prevent imported Japanese Famicom games from being played on it, but also to prevent the myriad of bootlegs available from being played on it as well.

This is before getting into the expanded capabilities provided by the Famicom Disk System, which we never got in the West. Fun fact: The expansion port on the bottom of your NES beneath that little hatch cover in the middle was never officially used for anything.

There is some more rambling to this effect in the now ancient post of mine here, including a showing off of the adapter in my Exitebike cart. As the story goes, for the US launch of the NES Nintendo could not produce enough game cartridges fast enough so their solution was to take existing stock of Japanese carts and bung them in cheap but official Famicom-to-NES adapters complete with a lockout chip on each, and package them all up in a US style cartridge which has a ton of extra room inside to begin with. Notably, if you've ever wondered when you were a kid why your Exitebike game pretends to have a save function, and where it's trying to save to, there's your answer: A Famicom tape recorder, which is a peripheral we never got in the US. But the game ROM is actually the Japanese version on a Japanese cart, in an adapter.

But if you're a dedicated enough nut, you can take that adapter out of your old Exitebike or Gyromite or whatever cart, and stick any Japanese Famicom (or pirate) cartridge in it, and mostly they'll just work with some minor limitations. The NES, for example, lacks the ability to accept the extra sound channel input that Famicom cartridges could provide via add-on chips.