Exactly all of this. I've mentioned the first point about add_header redefining instead of appending in a previous HN comment of mine: https://news.ycombinator.com/item?id=27253579. As mentioned in that comment, HAProxy's configuration is much more obvious, because it's procedural. You can read it from top to bottom and know what's happening in which order.

Disclosure: Community contributor to HAProxy, I help maintain HAProxy's issue tracker.