What a coincidence, some days ago I was reading some HN posts related to lighttpd and I found [1]. The link is dead and it has inappropriate content, so use arhive.org. The author doesn't go too much in detail of why nginx being purchased is a problem, but in how to configure lighttpd. And the first comment predicts the hypothetical case of F5 being problematic.
[1] https://news.ycombinator.com/item?id=19413901