Here's the thing.
The backstory for General Grievous was at first delegated to EU faggots because Lucas gave no fucks, and theirs was that he was a super warrior loosely based on the Predator, a real oldschool Shaka Zulu-motherfucker who got caught up in machinations by the Separatists, which basically echoed the story of Dengar.
The problem is, that's too cool. Lucas thought Grievous was a disposable character, not really more important than Nute Gunray. So Lucas took the reigns with The Clone Wars not only turning Grievous into a total wuss but also retconning his backstory, and since Filoni simply didn't give a fuck either they went on with it.
About him being potentially better than Vader as other anons have said, I think that's a much more touchy subject (y'know, it's fucking Darth Vader we are talking about), and I say this as someone who unironically has GG as their favorite Star Wars character.
Honestly, I have always had a dislike for Anakin. Not only because he's a terrible character all around but also because I can't make a reasonable connection between him and Vader, from design to characterization.