Database Sharding: When is it worth the complexity?

so, i've been thinking a lot about database scaling lately. we're hitting some performance bottlenecks with our main relational database, and sharding seems like the next logical step. but the operational overhead and complexity seem pretty big, you know? i'm curious to hear from those who have implemented sharding: at what scale or under what specific conditions did you find it absolutely necessary? what were the biggest headaches you encountered, and were the benefits worth it compared to other scaling strategies like read replicas or just upgrading the server.