In general I think that failures of all types of sustainer generally arise from ancillary components ( e.g. fuel pump, spark plugs, carburettor diaphragm etc) and electrical issues (e.g. relays, connections etc) and electric glider motors certainly have potential reliabilty vulnerabilities.

I think the reliability of a FES would be more reliable than conventional motors due to less parts. However, over the long term a concern I would have about FES would be water/dirt/debrie in the motor and electrical components, assuming that not all are shielded and even then the deterioration over time. I would think that all gliders and motors that are 20-30 yrs old have had wiring, rubber tubing, and other non corrosive components replaced. A solid aluminum/metal engine can be rebuilt and wiring/tubbing replaced. But what about FES? I know the batteries are not an issue due to life expectancy and replacement. But what is the life expectancy of FES motor? How about the build up of dirt and debris from aero tow? Can the FES be air blown off or washed with water?

I really like and think the FES is great and the future but long term maintenance is key to resale ability and market.