I think that the Japanese high command fell into the same trap as the Germans with tanks, believing that bigger was better. OK the Sherman was crap as a battle tank, undergunned (until the British stuck a 17Pdr. gun on one!!), poorly designed anti-armour ammo (M63, M72, etc.), ran on Av Gas, but it could be built in vast numbers, was reliable, easy to fix, etc.
A lot of small, fast tanks, with well trained crews, can be fielded everywhere where as a handful of resourcefully expensive, high tech, over complicated tanks can only be in one place at time.
Interestingly, German tank crews in 1941 were told to avoid direct combat with any of the 63 Soviet 5 turreted T35 land battleships. These giant beasts were left to occupy and dominate the small radius of territory they occupied whilst everything just went around them. The Russian learn a hard lesson form this in 1941 which they took notice of, hence the excelent T34, the best tank of the WW2.
The Shiano would have probally ssuffered the same fate. For the resources wasted on building it, the Japanese could have built 2 or maybe 3 conventional carriers. The US Jeep carrier programme proved to be the winning way.