DocPad provides a LOT of extensibility and dynamic capability, which means there's more up-front investment to learn DocPad well -- and deviating from the defaults while maintaining project robustness may be difficult.
Hexo built sites can be easily deployed to Github pages, Heroku, Openshift (custom cartridge needs to be setup) or any other custom solution (just copy over thepublic folder). Any deployment is as simple as editing the _config.yml file and running the hexo deploycommand
The entire Metalsmith core is currently ~350 lines of code in total, and the system is built around the simple idea of manipulating a dictionary-like set of file objects. This architecture makes writing plugins and understanding the Metalsmith code base super-easy.
The Metalsmith community is still fairly small compared to the more popular options. This results in a lack of learning resources and difficulty finding support from experienced users. However, a slack group has recently been started.