By expensive, not just capital costs, but costs around operating them - they weren't as reliably configurable, health-checkable, and instrumentable as we'd want, and Proxygen (and a later L4 load balancer) were.
Also the previous load balancers had constraints we weren't willing to accept - they required special connectivity to our networks, we could not use particular combinations of options, and we had to rely on vendors to solve problems that most of their customers were not encountering and/or able to detect.
Yeah, we are hitting the same wall right now and we've been going down the same route using HAProxy/Chef. Our plan is to put an API in front of it and treat it similar to an ELB. Are you still using hardware load balancers for SSL termination?
Also the previous load balancers had constraints we weren't willing to accept - they required special connectivity to our networks, we could not use particular combinations of options, and we had to rely on vendors to solve problems that most of their customers were not encountering and/or able to detect.