I hunt both pre-set tree and on the fly hunts depending where I am at. My favorite method is pre-set trees with screw in steps, I have at least 40 set up throughout my state. I have never had one break on me. IMO anyone who has one of these break on them has put them in the tree incorrectly. I have seen so many trees set up with them incorrectly, I almost want to start carrying notes around the woods when I'm scouting to warn people. A screw in step needs to have the L at the bottom portion where you stand touching the tree. If that L is not touching the tree, the step is going to bend under stress when it is stepped on. Eventually (and especially when it gets really cold), that step will break. The best way to put these steps in is using a tree step tool. I use the ultimate tree step tool: http://www.treesteptool.com/
I also don't know how a step came out of the tree. Maybe it was a dead tree, or the step was put back into a previously used hole, but in either case those are things to avoid.
Anyways, just some information from my experience using screw in steps. If you can afford them, the cranford ez screw in's are the easiest to use, but they are way more pricey.
X4