As I said before, it's not that it's too sad, after all I liked the Eclipse and that was ridiculously dark (though it is from a Seinen manga, but still), it's that he can never write a single female character that isn't either killed off the rip or survives due to being very beautiful/a MC like Nami, or strong but extremely deluded or ugly (Yamato and Big Mom).
Though I suppose Yamato would count as a good one, come to think of it, as she probably got the Oden shit from being locked up for so long and only having his journal to entertain her, but that's still only one good female character out of the hundreds that he's written.