I dont think 12% interest rate is insane. You can get that with a fiat investment. Banks charge 21% interest so 21%-12% there is plenty of room. The only thing I don't like is putting my coins in another persons address.
This is why I prefer masternodes to things like this. I want to be in control. This model is sustainable the thing is, is the owner sustainable and level headed or a moraless coward that runs off with all our funds?
The risk is very high I personally don't think 12% is high enough for the risk when there are offline investments I can make in fiat. If I was sure my bitcoin would be safe 100% then I would definitely use this service.
Nothing personal just my feelings on the subject.