Not totally sure about pre-50, but post 50 with all of our stuns and our pacify and two roots... it's safe if you know what you're doing. If root ever breaks just pacify, reroot and back off. Just always have at least one stun or pacify up if you do not have two roots up. Cower also fears mobs so that buys you even more time. There are so many safety nets in the Warlock arsenal, anyone who thinks it's crazy to solo is doing something wrong.