SSHKit is designed to solve a different problem than Capistrano. SSHKit is a toolkit for performing structured commands on groups of servers in a repeatable way.
It provides concurrency handling, sane error checking and control flow that would otherwise be difficult to achive with pure Net::SSH.
Since Capistrano v3.0, SSHKit is used by Capistrano to communicate with backend servers. Whilst Capistrano provides the structure for repeatable deployments.
It's in private Beta use, and the documentation could use more work, but this is open source, that's more or less how it works.