HAProxy 1.5 does a pretty great job with that stuff now. I agree that the configuration syntax can be difficult but it is very powerful.
There are some limitations of course, and I can see why you might want to use nginx for some routing. One of the more bizarre tricks I've had to use for more complex redirects (like non-www to www while properly injecting HSTS headers) involved sending the request to a backend that sent to a single frontend via a local port [1]. Hopefully that kludge will be fixed in 1.6.
I know its capable, but you have to understand we are working in a team environment where the expertise is primarily with nginx. It's also quite a lot easier to introduce nginx to a dev who has never used it before. Not every decision can or should be made solely based on technical capabilities.
Bizarre tricks are not something I am a fan of deploying to production and using as the foundation for our service. That is hardly a compelling argument. ;)
