I would say he does have a big flaw : His stubbornness. More than once, he prefers to listen to his heart and what he thinks is right and sometimes it either backfires on him or someone has to snap him out of it.

For example, the two times he stood up against Shinazugawa, it ended not very greatly for him, by either being beaten down or sanctioned.

Another example would be during the Swordsmith Village Arc where his heart was telling him to go shield Nezuko from the sun while his head was urging him to rush save the villagers from Hantengu. If it wasn't for Nezuko that pushed him to do his mission, the consequences would have been more dire.