when should you really consider a service mesh?

ive been seeing a lot of buzz around service meshes like Istio and Linkerd lately. they promise a lot of goodies like traffic management, observability, and security. but frankly, they also seem like a lot of operational overhead to set up and maintain. at what point does the complexity of a service mesh actually become worth it for a team or an application? are there specific metrics or use cases that make the investment clear, or is it often over-architected for simpler needs?