Any update on this issue?
On my app (KeyStage), I'm manually calling the renderblocks of all MIDI processor plugins in the AURenderObserverBlock while letting AVAudioEngine handle all instrument and effect audio units. But I'm having lots of issues with this approach: Getting EXC_BAD_ACCESS errors out of the blue, hitting high DSP for no apparent reason, etc.
I can't believe that Apple keeps promoting this AvAudioEngine framework and still hasn't resolved this major issue.