The single-arm pullover amplifies core activation to the extreme. There's an anti-rotation and rotary stability component that isn't present during the bilateral or isolateral versions of the pullover.
Add in the leg raise hold as I have my NFL athlete Fernando Velasco doing in this video and you'll find this to be one of the most challenging core exercises there is. It's also an incredible movement for exposing and addressing imbalances throughout the kinetic chain.
The "lumberjack squat" isn't what people in the logging industry head off to do behind a large poplar tree after having a hearty breakfast of bran flakes. Instead, it's a helluva' cool squatting movement that doesn't compress the spine.