Recently, I deployed GitLab CE on a VPS. GitLab (just in case you don't know), is an amazing piece of software used for version control. Now, GitLab has two pricing models - Community Edition, which is free, and Enterprise Edition, which is paid. But the catch is, GitLab EE's source is openly available. I won't argue with the fact that its a huge learning opportunity and stuff, but that makes no sense.
What I'm trying to say (write) is that there is nothing stopping any user from circumventing the software's licensing mechanisms.

Just in case you don't know what open core is, go Google it. This Wikipedia article might help.

Returning back to the case of GitLab, licensing is handled in a Gem named (very aptly named) gitlab-license. The Gem is meant to be open source but I didn't find its sources anywhere (but they do exist in your gem storage directory.) Even then, the calls are visible. But if modifying the calls doesn't seem elegant enough, a look at the gem's documentation, available freely online, is enough to craft your own license.

This leads me to the conclusion that.......well, open core is not a reliable way to monetize a product.

Note: This article is not a guide to circumvent GitLab's, or for that matter anything's license mechanisms.