https://outdoors.stackexchange.com/questions/18226/are-there-particular-grades-that-indoor-climbers-often-struggle-with
I really do not see how this is opinion based. Either there are grades that more people struggle with and others they find easy or there are not. The fact that no one has done the study to find out, doesn't make the question opinion based. In fact, I wanted to avoid people just giving their personal opinions and wanted an evidence based answer.