But I can't find anything to support the use case for highly available (multi-AZ), scalable, production infrastructure. Specifically, a unified and consistent cache across geos (AZs in the AWS case, since this seems to be targeted at S3).

Without it, you're increasing costs somewhere in your organization - cross-AZ networking costs, increased cache sizes in each AZ to be available, increased compute and cache coherency costs across AZs to ensure the caches are always in sync, etc etc.

Any insight from the authors on how they handle these issue on their production systems at scale?

PLEASE if someone from the team sees this - I would pay so much for a ephemeral object store using your same edge protocol (seen in the sensor example from your blog).

Cheers!