Just about any APU made by Airesearch/Garret (now owned by Honeywell) was overcomplicated and had little performance margin with the possible exception of their early efforts in the 727/737/DC9.
The 747 classic GTCP660 APU was terrible. The best thing Boeing ever did was install the PW901 in the 747-400. What a difference to ground cooling capability in hot weather an APU change made plus the reliability and time between removal increases.
The L1011 Hamilton Standard APU was a disaster. The best thing about it was it could be replaced in a very short time if required.
The 747 APU (all models) was available for pneumatics only if running on TO but it had to be manually shutdown by a certain altitude. This enabled a Packs Off take off in hot weather as far as the main engines were concerned with the APU providing air to one aircon pack for some cabin cooling. With some modifications to the APU control circuits and a ram scoop on the inlet door some 747 classic APU's were able to be started in flight below a set altitude but many long haul operators disabled this function and removed the ram scoop for drag reduction per a Boeing service bulletin. As far as I can recall the 747-400 APU was never available for inflight starting.