database sharding, when is it too much trouble

hey everyone, been thinking about database sharding lately. we're scaling up and the idea of partitioning our massive user table across multiple instances is on the table. on one hand, it promises better performance and availability. on the other hand, the operational overhead, complexity in queries that span shards, and the challenges with cross-shard transactions seem like a lot to handle. at what point does the pain of implementing and managing sharding outweigh the benefits for a growing application? what are your experiences or war stories with sharding?