Assuming it works the same way as their MySQL product, Vitess:

The usual way to run it is that you partition your db based on something like a user, so that single user gets a consistent DB, but anything cross-shard may not be.

I know when I worked at Block, Cashapp was using Vitess and getting cross-shard DB writes down and functioning correctly was one of the major blockers to adoption. (though I just did tls management for vitess and didn't write any workloads on top of it, so my impression might be a bit off)