I am not super familiar with the particulars of how a shinki is formed and what goes on between creation, but there are a few things to take notice of. For instance- when a soul is damaged or tainted by sin, their vessel form would not be in perfect condition. A good example would be if your God chooses to make a shinki out of a soul that just been attacked by akayashis and they transformed into a toaster oven. Their vessel form would have dents in it and may not function very well.