They were, in essence, the best WW1 warships ever made... except that they were deployed during WW2. The age of the dreadnought-style battleship was on its way out by this point and the era of aircraft carrier dominance had begun. Even if Musashi and Yamato had been deployed for key battles such as Midway and Guadalcanal, it's unlilkely they would have made much difference.
Yamato was deployed at Midway. She was part of the body of surface combatants (with one light carrier as escort) kept out of range for the surface action that Spruance wisely declined to permit. The deployment at Midway was a Rube Goldberg contraption that personifies everything that was wrong with IJN thinking in WW2; multiple formations scattered too far apart for mutual support and a requirement that the enemy do what you expect for victory to occur.
Neither ship was used at Guadalcanal for the same reason that the old American battleships weren't used: Neither side had sufficient tanker assets in theater to keep the old battle-wagons fueled. The USN deployed new design battleships (USS South Dakota, North Carolina, and Washington) but kept the Pearl Harbor survivors on the West Coast. The IJN used two older battle cruisers (Kirishima and Fuso) that weren't as fuel hungry as their bigger/newer cousins.
They were, in essence, the best WW1 warships ever made... except that they were deployed during WW2.
The biggest flaw with the IJN was their inferior fire control technology. This is evidenced both in surface actions (Samar being the best case study) and in the anti-aircraft role. The USN had radar directed fire control in 1942, for both surface targets and aircraft. The Japanese paid an extremely heavy price when attacking our ships with aircraft, the two carrier battles in the Guadalcanal campaign (Eastern Solomons and Santa Cruz) were Pyrrhic tactical victories at best, with most of the Japanese aircraft losses coming from AA fire.