[jryio]

I also do technical diligence, often times the blocker to sharding is the target company's tenancy model and schema. PK data type is certainly a blocker.

Definitely an issue, rarely the main one. You can work around integer PKs with composite keys or offset-based sharding schemes. What you can't easily fix is a schema with cross-tenant foreign keys, shared lookup tables, or a tenancy model that wasn't designed for data isolation from day one. Those are architectural decisions that require months of migration work.

UUIDs buy you flexibility, sure. But if your data model assumes everything lives in one database, the PK type is a sub-probem of your problems.