If you look at that picture of the JD LX6, I think you'll see that the top part of the linkage frame on the LX6 will keep it from flipping up. The top part of the frame will hit the back of the tractor before the main part of the brush hog will. Some brush hogs will be different, of course, and maybe allow the frame of the brush hog or PTO to bounce all the way up to the operator, but it's very likely a stretch for this to happen to most (think the PTO shaft will maybe even keep this from happening).
The linkage on my LX4 is identical to that on the LX6. It IS NOT flexible enough for the hills and uneven terrain that I run it over. I have a chain attached between tractor and LX4 most of the time. The chain is short enough to keep the LX4 from dropping off the side of a creek bank but will allow the blade to dig into the dirt when the wheel on the LX4 goes off the bank or into a hole. Unless one hits a rock, there's no problem. The hydraulics just slow the blade down (a big rock will, too). Maybe if you had a really high hp tractor, you might have to worry over this more.
I keep both the chain and the top link attached onto the top link part of the LX4; they fit side by side. I can very easily disconnect the chain from the tractor and swing the top link over and connect it for transport or for when I need the full weight of the LX4 back there to counteract weight in the FEL.
Ralph