So what’s stopping any DF from being exactly the same? The Gorosei are not going to give us full blown explanations for every DF
Common sense.
Why would the Nika fruit be special, then? Why was it hidden if "any DF is exactly the same"?
If every DF can just do anything based on the users imagination, then why is there an encyclopedia that knows exactly what they do and what they don't?
Why wasn't Whitebeard shooting laser beams or reality warping after his decades of experience and power? Or doing anything other than tremors at all.
So far, no DF has done anything that falls outside of what it should do (Law's operating room, Bonney's alternate futures and Luffy's Nika fruit do what is in their job description).
Yes, you can also use ancient zoans as an argument, but then you're treating Vegapunk's words as an absolute truth (not even his words, just your interpretation of them, since he didn't literally say what you're taking from them) while disregarding how those ancient zoans abilities were also explained in-story by a couple characters: "That's just what dinosaurs did" (in the world of One Piece). That's just Oda's idea of comedy.