I have a rooted Lolipop 5.1 phone and I was wondering if uninstalling built in apps safe like Lookout, Hangout or Google+.
Yes you can. You can manually delete them from the
/system/priv-app directory, or maybe using an system remover app.
Also, not all apps can be removed safely, for example, there are some core phone apps like
SystemUI. Do not delete them! If you're unsure, I suggest you to get information about it first.
You can use a app like System app remover for that. And most of the apps are safe to remove, as the mentioned app shows if the app to be removed is a core component.