Found some documentation at least giving an upper bound:
"App extensions should feel nimble and lightweight to users. Design your app extension to launch quickly, aiming for well under one second. An extension that launches too slowly is terminated by the system."
This is becoming slightly difficult to debug! The console logs for a successful launch are nearly identical to those of an unsuccessful launch. I have observed "AU failed to load in time" for both cases. Ultimately when my AU fails to load, I see an exclamation mark over the plugin item in the list and it's inactive in the audio chain.
The intermittent nature of the problem suggests a timing issue, and App Launch shows total time for launching my standalone app is 1.5s, which exceeds the upper bound in the docs. But there's not much help from the logs. I would have expected some difference when things work thus pointing me in the right direction.
Has anyone else ran into this problem?