Hi,
Thank you for your post.
If I am not mistaken the APU B747-400 is not certified for to be restarted in flight (same for the classics I guess). It is started on the ground and it will keep operating at a certain altitude, then it will shutdown (that's what I read in certain forum). I think there were 2 cases (vulcano ash encounter) where all engines flame-out occured and the restart was not eazy thing. In such case it seems (may be) even the autostart is not able to restart all engines In addition the B 747 is not fitted with the RAT (I guess we are little bit out of the subject of post which the reliability of the APU).
I hope I will receive more comments and opinions about the reliability of the APU's (old and the recent).
Thank you.
Best regards