There are some stability problems for these things - any VTOL, in fact, with its lifting means located low on the airframe - when operated near the ground. Enough to be a challenge even to modern controls. Main problem in those days would have been training GIs to operate them safely. That contradicted the main point of a flying jeep - namely NOT having to give specialized training. With helicopters, you definitely need specially trained pilots, but you also end up with much more efficient lifting systems.