Having flown RAF Tristars I can say the aircraft was a pleasure to operate as a Pilot or Flight Engineer, Some systems were a little harder to maintain but that was the price you paid for an all electric aircraft that was (in the 1980s) pushing the edge of the technological envelope, it was far more advanced that a 747 and at times, you paid for that edge.
In the 10 years I operated the Tristar I never had an APU problem (not saying that didn't happen to other guys), that may well have been due to the excellent Engineers we had at 216, I know those guys worked their ar**s off to keep our aircraft serviceable.
Like any aircraft there were pros and cons to its operation but the Tristar will always have a special place in my heart and if some dude wants a Pilot or a Flight Eng to fly the ones out in the States I have a list of guys available....