>This proved to be no better. Fortunately there is a safety release on it that will "pop" the head when it is overloaded. <
Sounds to me that you insist on overloading any stand mixer you've used, and then want to blame the machine.
I bought my Pro 600 about two years ago and have had no problems with it at all---other than the fact the 6 quart bowl isn't happy with light loads. I bake bread with it every week, most often using it for double recipes (i.e., two loaves) but often kick that up to three, and it doesn't even labor under that load of dough. Where talking about 12+ cups of flour in a machine rated for 14 cups. Let's say it really overloaded at 14, though (even though I have no reason to believe it would). That would be an over-rated claim of less than 15%---a long, long way from the 300-400% you becry.
It has a metal housing, and a built-in circuit breaker that prevents overload damage (mine has yet to break). And mine has never creeped, let alone walked, across a counter.
As BDL points out, each of us is just a datum in the statistical pool. But there are far more KA pro users pleased with their machines than not. So I suggest you look to your methods, and, perhaps, modify them.