Opik is an evaluation tool first. Arch is a proxy server built on top of Envoy so it borrows from a very robust observability source. They both are complimentary in many ways

Envoy is compatible with OTel out of the box. That's a big plus for observability. Plus Envoy is designed for high-load dataplane (in the request path worklaods) and used in every modern stack. There are several advantages on using Arch as the source of observability (traces, metrics, logs)