It can really be anything connected to them, be it their classification, limited powers, limited fruits due to their users etc.
I'll start:
I'm discontent with Candy, Wax and Poison fruit's classifications, wielded by Perospero, Mr. 3 and Magellan. I really don't see why they should be paramecia when they allow the user to create and control an element. I feel like it'd be better to make them logias, even if the user can't turn into them. I mean, Darkness fruit also lacks that trait, yet is a logia.
I'm also unhappy with the fact that all dinosaur fruits we saw at Onigashima were labeled as Dragon fruits for one reason, it being that a dragon category leaves no space for standard fruits, it can only be ancient or mythical fruits.
Adding onto that, the only actual dragon in Onigashima, Kaidou, didn't even have a dragon fruit. His fruit is actually Fish fruit.