I feel like "hard" is defined differently based on the player you're using. Songs that are a blast to play using Vegas or Eraser (which generally handle high traffic well) are much harder to play using Mono (which will just throw a ton of gray at you) or Puzzle players that depend more on precision.
For me, it's not the fastest/loudest/highest traffic songs that are the hardest, but the ones with really bouncy and/or syncopated rhythms. That creates a lot of undulation in the track and makes it a pain to see where you're going, versus just zooming downhill at a fairly consistent speed. Either that, or the speed will increase and decrease as the beat stops and starts, making it difficult to time it right when you need to hit specific blocks and avoid others. A good example of this is "Oxford Comma" by Vampire Weekend, which is a fairly easygoing song to listen to, but the syncopated verses have some real constipation going on in terms of track speed. Radiohead's "Idioteque" is similarly painful for this reason (despite being one of my favorite songs outside of the game).
For combination of high traffic and really jumpy rhythms, another Vampire Weekend song is a fun example - "Cousins". It'd be positively headache-inducing if it were much longer.