They're efficient at higher power levels but really not at small-power-generator level. An 8kw jet powered generator I saw recently used 0.5kw-hr per kg, which is 5x more than an equivalent one based on a traditional diesel engine. Their main advantage is high power for low mass of generator, but consume massive amount of fuels.
Not really because the power rating at which they start to make sense is in the multiple hundreds of kW. Cars don't need that much average continuous power while driving, on the order of 15-30kW depending on speed and grade. So a "more efficient than piston" turbine engine would be way too big and powerful and would cycle about once per hour. Overall excess mass and bad reliability due to frequent cycling. It's better to have a smaller engine that's sized for slightly over 15-30kW continuous duty. Realistically this will be a piston engine, or a fuel cell. I would love to use a turbine engine for my own application but the math doesn't work, unfortunately.
