Cilium also does Service IP routing, ala the kube-router feature you're referring to.
Thinking aloud, by "Service IP" routing do you mean that actual services can have external facing IPs? I had no idea kube-router supported that, maybe I'm on a version that's too old?
[EDIT] Looking back at the kube-router docs, I see no mention of what I was discussing above so I'm going to assume "Service IP" means the usual IPVS/LVS stuff that kube-router does. I find that feature doesn't really affect me directly as a cluster operator though... Though it's probably saving me some latency (I assume you can skip a DNS resolution or two?)
Anyway, I was just pointing out it is also available via Cilium. To put it another way, Cilium can replace kube-proxy, just as kube-router does. As well as doing all of the other mesh networking functionality that k8s needs.
Put a final way, I still don't know what kube-router does or roles it can take, that Cilium can't also do or provide. But I'm not deep on them and am mostly replying here in the hopes that you will fill in a missing gap in my knowledge. :). Thanks!