Agreed, writing a core system component that isn't going to change and be called from thousands of different places? By all means heavily test and fuzz and cove

Agreed, writing a core system component that isn't going to change and be called from thousands of different places? By all means heavily test and fuzz and cover the entire API surface. Writing some product feature that is probably going to be changed 1000 times and is at the edge of the system, waste of time. You'd be better off just having really good metrics and alerting for system degradation that measure actual business metrics, staggered canary roll outs, and easy rollbacks.