Reading through these comments and it seems the classic misconception of "Flashiness = Effectiveness" is strong here.
Garou's skill is incredibly flashy compared to Rex's. Dude has a laser-precise martial arts style and has by far some of the most insane fighting adaptation and survivability feats I've ever seen from a hand to hand character in any manga.
But that's the thing, though.
Half of that is all Flash.
Rex's Nanites can hyper-adapt to most abilities per WOG and also evidence against Quarry's laser and he was skilled enough to overpower beings capable of gravity, elemental, time, space, matter and etc with just his usual Machine-based constructs alone rather than Garou's better "Looking" feats of surviving hordes upon hordes of heroes with abilities nowhere near that problematic.
Also again, like it was pointed out here, his sword can slice through molecules itself. As for the debate of the molecular sword cutting itself against his Regen, not entirely sure about that one just yet.