I see no mention of foreign keys, or any other constraints, across shards. If, as I suspect, they're not implemented, it would still be useful but at the level of Spanner 10 years ago.
For example: point 08 says "Assign different tables or workloads to different shard groups" and point 02 says "Split hot shards as workloads grow". How do those interact ? Can a single table be split across multiple shards ? If so don't you need 2pc to enforce primary key constraints ?
The intro pages just read as AI slop.