Part of this article is relatively obvious, but there is an interesting tip about users switching between accounts.
Once a user logs into ChromeOS, they can add a second profile for a different Google Account and toggle between them. This is much like using profiles in Chrome for Windows or Mac. The article gives directions on how to block that and other user login controls.
Honestly, I'm not sure what the negative impact would be if users were allowed to login to your domain and then add an account that isn't in your domain. If they want to separate their work and personal lives, I consider that to be a good thing. The only advantage I can see is if your institution has a set of users that are being monitored (e.g. students who are minors or an employee on an improvement plan) and you need to prevent them from circumventing that monitoring. For example, if you use GoGuardian in a school setting, this could help.
At the moment, I'm not inclined to use the tips in the linked article. However, I'm leaving this link here in case it could benefit any on me else or in case I change my mind and want to do this in the future.