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.
Has your bench been stuck so long you've considered loading up the bar to your usual weight and just welding the damn plates on? If that's true, it's time to bust through that pesky plateau.