The network is definitely NOT reliable.
Istio bring tools to DevOps and SRE to manage network concerns on behalf of Dev teams letting then focus on delivering business value.
The sidecar patterns tackles microservices concerns in a uniform and declarative way.
“Istio can (and should) be adopted incrementally.” Christian Posta, Istio in Action
Benchmark other service mesh technologies: Linkerd is an option to consider also.
Service mesh maturity model: