What about the post snapping off above the bracket? Is this a real danger?
Or rather, the post coming loose in the bracket? It's a pinch bracket, no
holes actually get drilled in the post. It would have to squeeze the post
pretty tight to hold it in place. There is going to be a lot of pressure
at that point, but a 4x4 is pretty strong. OTOH, there is only 4-6 inches
of post in the bracket, instead of 2+ feet of post in a hole. I have no
experience with this thing, so not sure how well it would work. Mail box,
yes, 2 foot picket fence, sure, 6 foot fence? Hmm.

Oh, and any suggestions on how to drive them into the ground and have them
end up level? It would not take much to end up with posts that lean. I'm
tempted to pick up a half dozen and see how they work. If they do, great,
I saved a lot of trouble. If they don't, I've wasted 80 bucks or so -
these things are not cheap

I even thought of sinking them in cement. They would last a few hundred
years that way . If the bracket that holds the 4x4 was taller, say 8 or 10
inches or more, I'd think this was a great idea. But 4-5 incheas of bracket
to hold the post? That is where I'm uncertain.