Free version includes advertisement in HTTP response headers
In the free version, Caddy includes an Caddy-Sponsors header in all HTTP response for crediting their sponsors. This header cannot be removed, and is required as per the non-commercial EULA.
Caddy introduced commercial licenses in Sept 2017, making it non-free for commercial projects, although you can still build from source and compile/distribute the binaries under Apache 2.0. Furthermore, the pricing is steep when compared to Nginx.