I do see the point in using HAProxy as a loadbalancer, we use it too (in a different setup without SSL), but in the context of this tutorial it's usage doesn't

I do see the point in using HAProxy as a loadbalancer, we use it too (in a different setup without SSL), but in the context of this tutorial it's usage doesn't seem warranted when you're already running Nginx. We've had literally zero issues after switching to HTTP/2, and we had none when enabling SPDY early 2015 either. Since we have integrated Let's Encrypt with automatic renewal in our provisioning we've even gone back to old websites and retroactively enabled HTTPS just to benefit from HTTP/2 and the response from clients and users has been either positive or absent. The most annoying part was finding a good solution for the HTTP/2 / ALPN / NPN problem. With OpenSSL 1.0.1 Nginx will negotiate HTTP/2 just fine, but over NPN only (which Chrome is dropping soon). We wanted to avoid compiling our own Nginx with every OpenSSL update too. In the end settling on Debian unstable proved to be easy and by far stable enough to simply run Nginx.