I think all of my buddies with jet (JS) have had at least one failure.
None of these contraptions is 100% or even close.

None of the "sustainer" jet engines are much more than slightly upgraded "toy" R/C engines, with only partially developed Engine Control Units. I am not surprised at the reliability issues, as Bob Carlton and I used these in a jet powered Alisport Silent on the airshow circuit from 2005 to 2008. We carried four engines to make sure we had two operational at any time.

After switching to the PBS TJ-100, a true Mil-Spec turbine with total FADEC control, the problems disappeared. PBS has been manufacturing turbines since the sixties (or before), with much of their product geared toward Auxiliary Power Units for Soviet fighter aircraft and helicopters, (L-39 and L-59 Albatros and Mi-8, among others). After the collapse of the Soviet Union, the engine was redesigned into a pure turbojet, as well as a turboshaft and turboprop, with more sophisticated materials and a vastly enhanced computer controller.

PBS is a large company in the Czech Republic, with over 1,200 employees in a very modern manufacturing facility. They build turbine machinery for everything from hydroelectric power generators to small aircraft. Over 300 employees work in the aircraft division. More than 700 PBS TJ-100 engines are in the field, on everything from target drones to manned aircraft like Bob's airshow planes.

Since 2008, Bob has installed the PBS TJ-100 on his Salto airshow glider, a TesT-14 two seat glider (retractable engine configuration). Sonex Aircraft Inc. has sold over ten subSonex kits, with at least five flying and about five BD-5 aircraft are on the airshow circuit using the PBS-TJ100 engine. We are currently completing the installation of the PBS-TJ-100 in a third Arcus airframe.

In our personal experience with these aircraft, over 750 inflight engine starts have occurred with no failures. Of course, reliability comes from the engineering experience the PBS company has demonstrated, and that doesn't come cheap.

As Dave pointed out, reliability of the Arcus M powerplant and retraction system is somewhat "failure prone." The PBS-TJ-100 engine and Desert Aerospace retraction system has been extremely reliable, but it comes at a high price.