Nintendo releases Super Mario Maker so naturally since they're fake-nice like Disney they try to destroy youtube videos of romhacks.
Which is unfortunate since they can be pretty fun to watch.
Last chance to see these things:
https://www.youtube.com/watch?v=tmfvpiTWStA
I don't think Nintendo will win that in the end. Sure, they can issue a DMCA takedown, and that'll take it down for a month. But to push it longer than that they'd have to take it to court, and I really don't think they have anything that would win in court.
It's Mario assets? Yeah, companies have tried this before when, for example, a youtuber gave them a review they didn't like, or made a video about a glitch they didn't want public. The only thing that's different in this case is that it's a ROM hack, but ROM hacks are legal. (Distributing ROMs is illegal, but it's legal for you to make a backup of a game you already own, and legal for you to have an emulator).
The only thing I can think of that they have is that people who made the ROM hack broke the end user agreement. But that just voids the warranty as far as I know.