I believe the APU core on a B777 is a PT6 ?
Does size have any correlation with reliability - unless you mean that they are worked very close to, or at their maximum power capability all the time ?
As far as monitoring is concerned, did they still do that when the units were sold and leased back, I wonder. Not their problem any more, just phone up the new owners to send a other unit if one broke down. However, that attitude would not have helped us at all if we had an engine failure over the Atlantic, when having a 100% reliable APU fitted in the tail is quite important.