NGINX is a battle tested C high performances general purpose server, while Caddy looks more like a collection of diverse Go libraries bundled together. I don't see a lot of protocols implemented by Caddy itself. There is also the question of Go garbage collector performance and its over head, there is no GC pauses with C.
This is a great project though, the author is young and talented.
The Go GC with 1.5+ (and especially the work going on currently for 1.7) makes it all but unnoticeable for the majority of non-soft realtime (think HFT) systems:
For 99% of users, it is simply not noticeable anymore. Also note that I've worked at said HFT nanosecond latency types of firms the past 9 or so years.
This is a great project though, the author is young and talented.