How will the blades flap to equality?
The rings will be a royal pain to store in a hangar, unable to move the blades around to mesh the machines in tighter.
The ring will possess some unwanted gyroscopic properties, perhaps?
Balancing will be a trick, too, with 360 degrees of different places to shove a weight. But perhaps more accurate.
Will the ring be fitted when the blades are flapped up? Or level? Because when the spinning stops, the blades want to sag down, but the ring will stop that, causing some compression stresses - blades are usually only stressed for expansion stresses.