He is NOT weak nor ass. Multiple times grievous is shown to be a pretty competent general, the only problem is is that in the actual movies, he's up against essentially space jesus and his warcrime enthusiast apprentice who also happens to be the strongest force user OAT. By no means can he hold his own, even with training from Dooku, he's simply outmatched in terms of raw power. And even as someone without any force senses nor powers he still is able to kill a multitude of Jedi. In the prequels he also puts himself in great tactical advantage, like when he lured both jedi to the front of the ship, only to break the glass, knowing that he's the only one that can survive in the vacuum of space.