The powerplant is a VW derivative 4 cylinder 4-stroke air cooled engine, basically one stage removed from a Beetle. The basic problems were
(1) The build quality was appauling
(2) The engine over-cooled.
The consequence of this was that the regular thermal shock whenever the throttle was closed, tended to ****** up the engine. The rate of engine failure (and the RAF didn't do anything to the engine) was the reason for the solo student ban.
Why do the RAF get this when nobody else does? Quite simply, because the ATC do virtually nothing but circuits in it, whilst the aircraft was really designed (and is almost exclusively used in civil life) as a tourer / Self launching glider. So the regular thermal shock of flying 10 minute air cadet sorties all the time doesn't happen in civil use.
My opinion, privately but not publically expressed at the time, was that the lump of metal under the bonnet should be carefully removed, dropped down a hole, and replaced by a more sensible modern engine that can take that sort of cyclic use - such as a Rotax 503 or an HKS 700E.
So, it wasn't the RAF's fault insofar as they (we !) didn't do anything to mess up the aeroplane. It was the RAF's fault, in that they picked a totally unsuitable aircraft for the job of spending all day flying very short sorties.
G