I've been using screw-ins since they came out. Several different brands of one piece a two piece folders. I weigh about 250 and have never broken one.
I do always make sure it is screwed in as far as it will go, if that leaves the heal away from the tree I wedge something under the heel to take the flex out of the stress concentration of the thread. Folders, I always check the rivet at the start of the year and watch for excessive looseness. For what it's worth rolled threads such as on the "Deer Me" are stronger than cut threads. This also means the shank is smaller dia. than the thread and lets it screw into the tree further and easier.. I have a few of the ones like you broke, and I guess the little welded gusset is a tip off that the steel is cheap as they had to gusset it to keep it from bending there. (Others don't) Deer-Me's are smaller dia and stronger, and with the rubber coated foot, don't grab my hand or glove near as bad as the knurled ones, while screwing them in..(I try and keep a piece of plastic pipe handy for screwing them in)
One of the guys who rifle hunts one of the same properties I do, uses a 2 piece folder that I would not trust for anything.. I think they still have a K-Mart sticker on them and the rivets are just too weak..
I guess I am trying to say there is some junk out there I would not use, but buy wisely use wisely and I doubt you'll have many problems.