Emulators are legal as Ian pointed out. But distributing ROMs or downloading them is probably not.
I wouldn't say every Nintendo game is illegal to use ROMs, as wouldn't that depend on the individual companies who made the games more than just Nintendo? Unless there was some contract that said once they sold their game through their system it can't be distributed any other way.
I've got a few ROMs for an Amiga emulator, good luck finding a company still around that cares. And as soon as I find my 2nd floppy drive I'll be able to make my own ROMs from the hundreds of old disks I have. I'd argue that ones I create from original disks is legal for me, I don't plan to host them online. But as for the few I've downloaded, despite them being over 20 years old, may still technically be illegal. I'm not too sure since most of the companies aren't around. Plus, I think the term "abandonware" had a lower year requirement back then than it does today.
It case it wasn't clear, I stand by Jeku's claim.
"Only the educated are free" ~Epictetus
"Imagination is more important than knowledge..." ~Einstein