They hunt down emulators of their current console (Switch at the moment) as they believe it hurts their bottom line. They don't give a damn about Wii or 3DS emulators

They don't necessarily bother with older stuff. The focus is to (unethically) mess with the stuff that is currently hurting their bottom line, which is why retro only occasionally gets shafted when they repack a 20 year old game, unadapted in an emulator, for twenty bucks. Although, rom hacks also get routinely fucked for whatever reason. That stuff can't even be making any monetary difference, so I'm assuming they just choose to be dipshits as a goal in itself.

The profit stuff is genuinely shady but they sure aren't just going for those. They've DMCA'd rom hacks and stuff that just weren't doing anything like that

Technically Yuzu was, allegedly, using something proprietary to run properly. I think the same is true of Ryujinx, although not sure. Now I'm not sure if it's true, given they settled out of court, and honestly we do know the REAL reason is the bottom line but legally they can't use that argument. Ironically I don't think it's even illegal to profit off an emulator, lol.

That said, emulators are legal in the same way a bong is legal while weed might not be. It's technically illegal to use roms/isos you yourself didn't rip, even if you have the cartridge or disk (and thus, license) of that game because it's not a copy of your cartridge or disk, but it'd be pretty annoying to prove you didn't rip it yourself as you'd probably need to go into the code to see if there's a unique identifier. I'm not sure if that's ever been tested in court, though I wouldn't want to try it.