It's probably fair to say that General Grievous didn't quite live up to the promise of his marketing, what with his bizarre respiratory problems and how easily he was bested by Obi-Wan Kenobi, but relatively speaking he's still an extremely potent warrior.
Grievous also has no Force sensitivity, but he is exceptionally well-trained in the use of lightsabers, and if his quad-wielding ability wasn't terrifying enough, each of his sabers is actually a "trophy" that formerly belonged to a Jedi he defeated in combat.
Grievous' cybernetic upgrades give him a major edge against the Jedi, though the fact he remains partially organic is his one major flaw, which ultimately led to his downfall. Still, the guy had at least four confirmed Jedi kills under his belt (and much more in the Legends continuity), so he's certainly not to be trifled with.
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