The FES has an apparent advantage in higher reliability and, perhaps, lower cost, as well as less maintenance. An engine powered sustainer has a potentially longer self-retrieve distance. Jet powered sustainers have an uncertain reliability. These motors must be spun up to a very high RPM for starting. The motors themselves are not certified for aircraft use. I know of one pilot who landed out in a very bad area when his jet failed to start after 3 attempts (miraculously neither the glider or pilot suffered any damage).
I personally won't have a sustainer. I fly a lot in Utah and Nevada, and the sustainer probably won't get me over the mountain ranges I have to cross, or even get me to a landable field that may be 50+ miles away. But most pilots don't fly in this challenging environment.

Tom