Lack of Scientific Defense!
It occurs to me that there is absolutely no reason why people/islands can't regularly have defenses against Devil Fruit users.
Haki is one thing.. but if someone only has a Devil Fruit, then all it takes is some sea water to turn them into a useless lump.
So... Why hasn't any of the kingdoms installed defensive technology designed specifically to counter Devil Fruit users? They're obviously dangerous.
It's not like they really need Sea Stone. It's rare. You know what's not rare? The sea water. It's literally everywhere. They will never run out of it.
So... I think that's a flaw in the story. Plot protects regular Devil Fruit users from just being hosed down or from having buckets of sea water thrown on them.
Being hosed and throwing buckets is the LEAST they could do. To be honest, every island should have a giant sprinkler installed just in case a Devil Fruit user starts going nuts.
All you would have to do is pull an alarm... and BOOM no more Devil Fruit user threat. He's a useless lump, now.
Note: For a God of Scientific Defense, Saturn really sucks.