Well coming from experience, I can tell you that a lionfish sting is no walk in the park. I have been stung four times, and all four times I have had the sting under control in 15 minutes without any medicine or hospital visit. All you need to do is run the infected area under very hot water. The first time I was stung, I did not realize it, and the area I was stung swelled up like a golfball, but hot water controlled it from spreading through my body.
On a positive note, I have never been stung by one of my two lions that I have in my tank (I have had my volitan over a year now, with the dwarf being a recent addition). Like stated, they are not aggressive and will not just come at you because they are aggressive (like a trigger). I even hand feed my lions, without any problems whatsoever. My new baby dwarf is so small that when I am giving him ghost shrimp, I let him bite the ghost shrimp out of my fingers so that no other fish get it.