I haven't checked back in a while, but I've pretty much always considered them to be overly reliant on either RNG, NG+, star knightess, or drugs, to be beatable before 80 or 100 days. I specify 80 regarding Belphegor because of the adventurer/s that die if you aren't fast enough. Belphegor IMO is the larger issue, because there is only so much you can do to counter play magic damage.
I completely understand that the game needs to strike a balance between corrupted Aura and pure Aura, but I think pure Aura runs are still unable to achieve certain things. Removing the collar is a last resort for when you've screwed up or fought something early, for instance, but against an enemy like Belphegor, if you want to beat it without NG+ you have to choose corrupting options.