As a load balancer, NGINX is subpar in almost every feature comparison, especially at the open-source (free to use) tier.

HAProxy gives you the following that are musts for load balancing (in my opinion), that NGINX does not, at least not easily:

1) A HTML (or JSON) stats page that precisely and completely tells you what’s going on at a high-level. A visit to this during outages is often all that’s required.

2) Support for DNS (and other) discovery mechanisms in a flexible way. (This is paid in NGINX)

3) Active health checks (also paid in NGINX)

4) The ACL system, while somewhat difficult to learn, is amazingly powerful.

5) Flexible L7 retries are brilliant.

We replaced NGINX with HAProxy and eliminated a whole class of bugs, micro-outages, and annoyances just by following HAProxy’s best practices.

I still use NGINX when I need a static web file server, though. :)