This got me curious and while a quick search didn't give me much info on what kind of "howitzer" these junks had, this similar naval gun had an optimistic fire rate of 5 rounds per minute and was less effective against agile targets. https://en.wikipedia.org/wiki/20_cm/12_short_naval_gun

So my guess would be light, slow, inaccurate gun against highly maneuverable bazookas. It seems to me like a bazooka vs tank situation. Tanks are more powerful but a well positioned bazooka will kill a tank.