What other reasons would an invasion of the Japanese islands be a concern for American military planners

One of the biggest reasons why the US was hesitant about invading mainland Japan was due to their geography. This is because a very small portion of their coastline was suitable for military conditions. Since this was the case, the US only had a very specific spots where they could invade. Japan, knowing how difficult the terrain is on the coast, knew this information and was well equipped to deal with any invasion that may occur.

This is one of the reasons why the US decides not to do a land based invasion, as the Japanese would be well prepared and because the invasion would cost significant casualties for the Allied powers.