It's safe in that there are sufficient guarantees in place to ensure that you can't lose all of your capital.
However whether the predicted returns will materialise depends upon the wholesale prices that can be achieved for the energy generated (and upon the operational costs required for doing so). So there's no actual guarantee of the predicted 10% p.a..
Further, you need to remember that the 10% p.a. figure is based upon a simple interest calculation. (i.e. you put your money in and, hopefully, get 50% more back in total over a 5-year period). That's equivalent to a little under 8.5% p.a. on a compound interest basis (which still sounds very attractive though).
https://aegispowerplc.files.wordpress.com/2013/08/aegis-wind-bond-faqs-v1.pdf