Don't be too sure. Remember that Mandalore was fighting a severely weakened Republic. General Grievous had to contend with the Republic that could churn out ships and soldiers very quickly.
Mandalore just doesn't have the numbers to keep up with Grievous. The CIS armies were able to out number the Clone Army by a lot. And the Clone Army was roughly one billion soldiers. Not to mention the CIS fleet was able to keep up with the production capabilities of the Kuat Drive Yards. It wasn't until near the end of the war that the leadership of the Jedi and the industry of the Galactic Republic that the CIS was defeated. Not to mention the failed attack on Coruscant.
Mandalore just doesn't have the industry that the Galactic Republic had. While his soldiers and ships are better (arguably), he doesn't have the ability to match the production capabilities that General Grievous has. While I am a firm believer in Quality>Quantity, in this case Quantity wins. Mandalore just can't keep up.
Well said. In the end, numbers and industry win it for Greivous.