StarWars.com on General Grievous [Legends]
Thread preface: This is Legends only i.e. no material that comes after April 2014. The Obi-Wan databank from the old Star Wars website that was included in Leland Chee's holocron, states the following about Grievous:
"The droid general had been trained in lightsaber combat by Count Dooku. Grievous lacked the finesses of a master swordsman, and instead used brute and whirlwind tactics against Kenobi. Grievous' artificial anatomy allowed him to wield four lightsabers at once, and spin them like deadly buzzsaws. But, since he could not use the Force, Kenobi was able to anticipate his blows, and counter them. Obi-Wan sheared off several of Grievous' lightsaber hands, and forced the General to flee." -- StarWars.com Databank: Obi-Wan Kenobi.
This quote dates all the way back to 2005. It says that Grievous lacks the finesses of a master swordsman, but relies on his extremely unconventional fighting style and constantly spam it at Kenobi i,e. "brute and whirlwind tactics" rather than pure lightsaber skills, simply because he isn't a master swordsman at all, but a pressure fighter. Obi-Wan was able to anticipate Grievous's attacks through precognition and counter them with his own. This is also evidence of the saying "styles make fights," because although Grievous isn't a master swordsman, he makes up for that with his brute strength and whirlwind tactics, which are clearly really unconventional and randomized, given that he has been able to challenge many great Jedi of his era.