No, the RIAA notice is on pretty solid ground in the U.S.

You'll note that organizations like the EFF haven't weighed in on this like they normally do when content owners get overzealous with enforcing IP controls, nor have many of the usual IP law commentators.

And more importantly, the youtube-dl creators would have already filed a response if the RIAA notice was as week as so many people in this thread have claimed; they've had a week to do so. The fact that they've yet to respond indicates that either they or the lawyers they've spent the last week talking to are having difficulty finding a response to the RIAA's notice. (This doesn't mean that they won't file a notice, just that the issue is not a piece of cake like so many people on HN believe.)