I dug back through our design docs from when we designed this, and we largely chose not to use Cloud Spanner due to unknowns. We were more confident we could predict the read and write performance in BigTable (especially due to the constraints you get when you drop relational features).

To be honest, it never even made it onto our radar, not for any particular reason though :)

IIRC, Spanner relies on precise timing to make certain guarantees, which is definitely relevant to our use case. I wonder how its write performance would stack up against Postgres and Bigtable.