Shawn pretty much hit it on the head.  The lacquer is the issue when it gets hot.  I've had the most problem with Wolf on this.  I will say I have shot probably thousands of rounds of Tula .223 without problems.  I believe they use a polymer coating as does some of the newer wolf.  Steel case doesn't have the negative stigma like it used to.  Especially when you consider that companies like Hornady make a training version of their 75gr TAP ammo that is the exact same bullet in a steel case.  Great accuracy, same POI as my defensive TAP round, and reliable.  I don't hesitate to shoot steel (but cleaning does reduce any chance of issue).