That grapple should work pretty well if you've got the money to spend on it. To me, it's just as easy to fabricate a unit such as ToadHill's, and wrap a chain around the log and go. Maybe not quite as fast, but a heckuva lot cheaper, and still gets the job done quite effectively.
Ever since my Dad and I started dragging logs way back in the early 80's, we only used the 3PH drawbar and a chain, that's it. No problems whatsoever, other than occasionally hooking up to too much log with not enough tractor. You know what happens then? Nothing... you don't go anywhere. Cut the log in half, and away you go. Never flipped anything yet, and we've dragged a LOT of logs, up hills, down hills, through creeks, you name it, we've probably done it or something similar.
As far as CG (and if you want to get technical, it's NOT Center of Gravity, but pull point) with that grapple and Toad's device, yes, it will be higher. However, what the SafetyPolice don't realize or take into account, is that if you can raise the end of the log at the tractor, so that the log is only dragging at the rear end, you reduce the amount of power you need to drag that log, as the majority of it is on wheels now and not dragging on the ground. Now you have transformed most of your load into a carried load, just like using a carryall, with just a very small tractive load. If you've got the end of the log raised up, then most everything you could get hung up on with the end of that log will go under the end of the log, and the log will slide right over it.
OK, so now what if you hook up to a big one, and your front wheels lift a bit? No big deal there either, if you've got your brain engaged, like Toad said. Stop pulling. Add some front end weight. Most of us have FEL's, so just get a scoop of dirt, come back and drag the log. Or cut it off so it's not quite as big of a load, and make 2 trips.
Even if you do raise the front end, the tractor will not flip over using Toad's device... as the front comes up, the back goes down, removing the weight of the log from the rear wheels of the tractor. When this happens, you don't have nearly as much traction, the wheels start to slip, and the front end will stop raising, once the pull point lowers to the plane of the axle. With Toad's device (and the grapple), that happens very quickly, and the front does not raise very far. And once it gets there, it's very difficult for it to go any further. Toad's will go higher than the grapple, but neither one will allow the tractor to flip.
So, why does a tractor flip then when when you hook up high on the tractor, like at the center link attach point. Simple. Just look at how far the tractor has to rotate around the axle for the pull point to reach the plane of the axle!! The nose of the tractor can raise up to 90 degrees and beyond and it still hasn't gotten there yet. It's all about geometry, and available traction.
