One of the things about nginx is the lack of an organised community - for e.g. there is not even an official repository for nginx (
http://marc.info/?l=nginx&m=122153991029203&w=2). there are simply some mirrors of people who maintain a patch-based tree (
http://mdounin.ru/hg/nginx-vendor-current).
There is no bug-tracker (!!), just a wiki page (
http://wiki.nginx.org//NginxBugs) and as someone mentioned (
http://www.wikivs.com/wiki/Lighttpd_vs_nginx), very less activity on IRC.
It comes down to the original issue of Linus Torvalds, Ingo Molnar and Con Kolivar: do you have a clear roadmap of where the architecture is going, vs a very cool technology that had a lot of support and was no doubt popular.
I am in no way commenting on the technology behind nginx, but as an architect, making a deployment decision that is going to take hell to change later, I would be very concerned.