I guess I'll throw in fast chords on the piano. Not many chords on the piano require much dexterity to play (with a few exceptions of course) and playing single notes fast doesn't take much work aside from initially getting some technique down. When you combine the two though, it becomes murder. Like if you have to do some Liszt-esque crazy chromatic scales at 200bpm in sixths. Technique doesn't help much since you can't position your hand in a way other than "hit chord, jump to next chord in split second."

Pretty much, if a song has some fast part with harmony in one hand, it's the hardest part of the song.