Well, the teeth would need a counteractive pressure to allow stuff to be chopped. Chopping through rock would be easy; chopping through water might be harder to do because it’s softer.
If there is a way to quickly HARDEN whatever material is about to be shredded, the MOMENT before the teeth strike, it might work though. Maybe a canister of liquid nitrogen or something would be enough. Squirt, freeze, shred, move on.
Suction will harden the facing area of the ‘whatever’ that needs to be shredded. The pressure closest to the blades would be less than the pressure BEHIND the THING-TO-BE-SHREDDED. It will ‘push’ the stuff in, a virtual “hard wall” behind… and SHREDORAMICITY. … yeah… shred-o-rami city… I degraded myself slightly there but yet, I am not ashamed.