I believe it was said by Chopper that they distort the wavelength of devil fruits, and not specifically only Zoan. However, Zoan fruits are the only ones with limitation. There's no way Luffy eating one would have any major benefits, especially considering the detrimental aftereffects.
Yet, considering the devil fruit goes out of control when eating three, any human eating three will have no consciousness. Does this mean there's a devil inside losing control? They are given the name of Devil Fruit, and there was even an explanation of what happens when eating two fruits, with devils implemented, but do we truly know if there are devils inside? They could just be distortions of the properties of the (already-screwed-up) world's physics, like some kind of dimensional thing (which could be analogous to devils). So it could be that dimensional distortion going out of control with the human as the gateway. So it would basically come down to DF users losing consciousness and their powers being used constantly, regardless of my devil-related tangent.
It really depends on the fruit. Robin might be able to sprout a million hands. Miss Valentine might be able to go to a million pounds. Enel might be able to go to 5 trillion volts. So yeah. Dangerous. Yet all Luffy could do is constantly stretch, or maybe be able to do some extreme form of tekkai.
Time for a bit of sort of off-topic. I think Oda doesn't want to focus too much on the supernatural, so while Devil Fruits might be explained in the future, I don't think it's going to stem any farther than that. The story will always be about people overcoming obstacles and living for their treasure, not some boring tangent for an overly long period of time.
OP