They're completely different products for different uses -- Cilium is a overlay networking solution (so, more than one program working together) for use inside container orchestration frameworks like Kubernetes/Mesos/Nomad, where as HAProxy/Traefik are applications that do reverse proxying and load balancing.

Cilium may happen to do some proxying and balancing but you dont do things like "add a backend to Cilium" -- you "add a backend" to Kubernetes, Cilium is handling your networking, so it sees that, and ensure your frontend (which is already deployed let's say) inside the same kubernetes cluster can speak to the backend when it reaches out for it.

HAProxy is a TCP-only reverse proxy and load balancer similar to NGINX.

Traefik is a reverse proxy and load balancer (currently HTTP only I believe) built relatively recently with modern features like a HTTP administration interface, OpenTracing compliant (IIRC) request tracing, let's encrypt for certs, and automatic polling of a container orchestration layer (if you have one) to populate backends/frontends.