[THIS IS ALL STRICTLY IMO]
Lately I've been noticing that GH, RB [and more specifically GHIII] is being touted as being some of the hardest music games ever. Not only that, but GH/RB is all that anybody considers "music game genre"?
First off, does DJMAX, DDR, Pump it Up, In the Groove, Precussion Freaks, Guitar Freaks, Drummania, beatmaniaIIDX, Pop'n Music, etc not exist all of a sudden? I mean, I remember when that video in some Japanese arcade of some absurd Pop'n Music song went viral, now it seems like people are too lazy to do some basic research. Case-in-point: When Konami released beatmania in the U.S for the first time since 5-Key 1st Mix / HipHopMania, when X-Play on G4 called it a ripoff of Guitar Hero - despite that it predated Guitar Hero by 8 years.
Now the main point of the rant: Nearly every other music game out there is harder than Rock Band and Guitar Hero - an assertion I make based on its game mechanics, and by observation. I'm going to use beatmaniaIIDX for comparison.
For one, Guitar Hero and Rock Band seem to have a very binary note registering system - either you hit it, or you miss it [with no degree in between] with a rather wide margin for error. IIDX OTOH has rather strict timing windows, and many degrees of "hit the note" - from the perfect timing [flashing GREAT, or "Just Great"] to poor timing ["Poor"] - in IIDX, 100% actually means 100% hit within the "JUST GREAT" timing window - though the letter grade AAA is more lenient than that [for good reason, IMO].
Others cite the # of notes in a note chart as a spectacle of difficulty. One thing I find faulty with this argument is that the songs are longer than in most other music games. 3 - 5, even 10 minute songs in GH / RB for the most part whereas the longest song in a game like beatmaniaIIDX is 2:30 in change on average. 3,722 note Full Combo in TTFATF [7:22 long] doesn't seem as impressive as doing nearly 3/4 of that in a 3rd of the time, like you can with a few of the IIDX songs. Mei [on Another] is just about 2 minutes long, 2,000 notes. If it was about 4x longer [almost matching with TTFATF] with that many notes per 2 minutes, forget about it. The ration of notes to time is far greater with IIDX, IMO.
While there is /a/ challenge, GH/RB is far from the hardest music game out there.