The Seiran was a competent aircraft trapped in a failing strategy

How good was the M6A Seiran as a late-war attack aircraft?

It was a respectable, reliable evolutionary design, but it belonged to a specialized program that arrived too late to alter Japan's strategic position.

The Seiran developed from earlier Japanese floatplane work and was not inherently a poor aircraft. Its lineage produced an evolutionary and reliable attack aircraft suited to the submarine hangar requirement. The more serious issue was the highly specialized I-400 concept entering a war already dominated by American air and sea power. Technical maturity did not guarantee a useful operational niche.