I have the Five SEs too without NASTY. Echo what everyone else says. Little to no issues with shoehorning once you get the straps dialed in. I do each side with one hand. I imagine it's even easier with NASTY.
Out of the box never an issue with the high backs not staying down. Never tried strapping in on the lift. It takes only a few seconds on the ground.
The only issue I've encountered is the straps overtightening. I fixed this with electrical tape on the underside of the ladder. Flow's locking buckle needs improvement. Otherwise very nice product.