I don't own either one of these hitches, (though I think I'd like to), and so haven't inspected one of them that closely.
I take it the geometry of the 4-bar linkage is not adjustable? In other words, it's a fixed projection amount? You can't dial it in to put the pivot point where you want it?
A lot of Fifth Wheel setups I've looked at have had the hitch point actually about a foot ahead of the axle.
To me, having it behind the axle just isn't as good. Certainly, any amount you can move it forward of the ball is helpful. But I would want to have it at the axle "pumpkin" at a minimum, and preferably a foot ahead of that.
I've never tried to make a hitch, but I have read quite a few books on auto suspension design and using long arm/short arm "double wishbone" suspensions to move the roll center as low as possible. These hitches are the same science, just rotated 90 degrees.
They really are a good idea, at least to my "P.E. in Mechanical Engineering Geek Brain" way of thinking
Although, I'm still not convinced that a modern version of the "Slimp Dolly" wouldn't be a great and wonderful thing too. I know there are at least two of them being produced right now. If you had one of those with a little electric motor on it, so that when you went to park the rig, you uncouple the tow vehicle and just back it on its own with the dolly....that could be pretty handy for getting into tight spots
Not to muddy the waters though...just got me thinking here.
See ya on the road,