I wonder what the deflection is on the center of, say, a 10' wide 6x6 supported from the floor with a suspended load of, say, an Oliver 20" Planer or 12" Jointer so say one ton.
There are some lighter and cheaper commercial gantry cranes around, from Northern Tool or Rand I think. If you had the room and the cash for them, the big ones are clearly the right answer--they roll; you could pick up your car if you wanted <g>. Pallets of plywood or a couple hundred BF of lumber steel-banded would be nothing. The engine hoists are, yes, unstable.
Or a Bobcat.