I have had two harnesses with an overgirth like that for my guys and found that if the cart is not perfectly balanced I got ALOT of up and down movement with the shafts. Enough to sore them up. I went back to wrap straps and the guys were much happier. Of course every cart and harness setup is individual so your freind may have good luck with it. I personally hate a set up like that (sorry for the strong words but I spent many many hours and $ trying to get them to work with no luck so I'm sorta soured on them). I love my wrap straps!
Another thing is the back and forth motion. Wrap straps help a little (along with the breeching) to keep the cart from pushing forward when changing speed. Those buckle down shaft loops just let the shaft hang free and don't secure the shaft at all. Sometime you have to use "thimbles" on the shaft ends to keep them from riding foreward. To me that's just one more added thing to fuss with. But like I said, this is all just my personal preference.