A design deficience? I wouldn't go that far. Just because it was designed that way doesn't make it a bad design. Deere is know for some pretty darn good equipment.. The geometry of their drag mowers, shields and gear box setups at the fron, keeping in mind driveshaft angle and geometry.. there simply isn't much room left for a torque limiter right at the rear coupling point. That's why it is at the front of the pto shaft.
Since the tractor has a pto stub shield.. I don't see the issue.
Another thing.. with a large implement.. you are probbaly ( should be ) using a large tractor. Due to that geometry, the operator is not in a direct line of fire from flying peices in the event of a torque limiter failure. I.E... If I look back, I can't see the pto stub/driveline connection point on my larger tractors... thus no direct line for shrapnel. If it were mounted farther back there would be some chance.. but still low, agan.. due tot he geometry and size of the tractor. I have a feeling this would all be very different on a SCUT or even CUT.. where.. the torque limiter being ont he implement side gets it farther way from the operator. And due to geometry and size.. the operator is probably in a bad line of fire no mattter where the torque limiter is in the drive train... so they opted for the lesser of 2 evils.
Ergonomically. the torque limiter being on the tractor end does make for a few extra pounds to lift when attaching the pto shaft... that said though, those big metric #6 and larger drive shafts are already a bear to lift. A few extra pounds doesn't mean much.
Soundguy