This seems hard to do in Vitess or PlanetScale given the technically READ UNCOMMITTED isolation [1] when cross shard, and, in scaled deployments, will still require experimental 2PC [2] cross shard transaction. So, like, yeah, if you had serializable isolation, then transactions might save you so long as your code isn't buggy, but, literally the reason the system doesn't implement them is because it doesn't have isolated transactions.
[1]: https://news.ycombinator.com/item?id=22170416#22177783 [2]: https://vitess.io/docs/13.0/reference/features/two-phase-com...