So while reading through Houses of Hermes: Mystery Cults, I came upon the Merinita illusion mystery, Perpetuity. This major mystery allows 3 new durations that have the potential to last permanently, so long as some condition is met. Each of the 3 durations - Might, Aura, and Hidden - have some caveat that will result in the effect ending, and all 3 of them have the same level modifier as Year (+4) and all require the spell to be a Ritual, which means they consume vis. Additionally, the last statement of the mystery indicates that most (but not all) Merinita magi who learn this mystery also gain the Unnatural Magic flaw.
But why even take this mystery? By rules as written, all Creo spells can already be made permanent with a Ritual Momentary (+0 magnitude) spell.
So let's say you want a permanent spell that creates the illusion of an animated butler for your covenant. You could either use the Aura duration and add a +20 spell levels to the Ritual (and costing 4 extra vis), while also having the caveat that should the aura ever be removed from the area that the entire spell to be dispelled. Or you could just make it permanent by changing the duration to Momentary and add 0 extra spell levels, with 4 less vis cost, and not worrying about any such contingent requirements.
Opting to take the Unnatural Magic major flaw seems bananas to me, because it sounds like you're just making it harder on yourself to make permanent spells.
Am I missing something here? Perpetuity counts as a major virtue, so I would like to imagine that it's doing something incredibly powerful here, but I just can't see what it is.