Won't Get Fooled Again has a section that's just the first guitar sustain and can't be leveled up at all. I wonder if there are any cases like that that make gold stars literally impossible?
I like challenging gold-star cutoffs, but the second that it reaches the point where you have to learn the song specifically to get gold stars is the point where I lose interest. Gold stars, for me, is a sign of a really good sightread, not an accomplishment after repetitive practice.
They simply could not have playtested every single song for GSability.
I am sure sooner than later someone will declare something un-GSable and them we will see.
What's weird about the game mechanics for RBB, though, is that it is not linear. Your first little mistake costs a lot more than your second, or third, etc, because your multiplier keeps increasing. So it's kind of like... if you get gold stars, you did well, but if you mess up at all, you will just barely get five stars, if that.