For hard stuff I need two things to hit it. First, the notes have to be spaced out enough and be scrolling fast enough so that I can read the pattern/rhythm on the fly, and second I need to see 'em early enough to read them. The spacing and scroll rate was always handled fine by Breakneck Speed in RB2, even though I would've liked more options than just a whopping two speeds... but the way the neck is more transparent, it becomes infuriatingly difficult to read the notes in time and very easy to lose some completely to the flashy background crap.
Haven't played Guitar Hero in a while, but it seemed like they were doing a much better job right from the get-go when the franchises switched. GH3 had like 5 or 6 speeds, and the fretboards weren't bad.
With pro keyboards, if anything, I'd want a slower scroll speed.