[Sorry about the lack of updates! I actually have only been updating my autobiography since I started that. Now that that's completed I will probably update weekly to Animation Theories and Son of Time. Basically, every Friday for Animation Theories and every Monday starting the week after next for Son of Time. A lot of research is needed for both, but this theory is a lot different than others because I've worked on it for years.]
Okay, for this theory I'm going to thank the guys at r/OnePiece for helping me finish it after 4 years. So, each person we see in the One Piece manga and anime that has eaten a devil fruit actually seems to take one of three positions in relation to their devil fruit. Oddly, they are divided between the three major types pretty evenly. So for this theory, I will use the three different types as the basis for each point. The three types for people who don't know are Paramecia, Logia, and Zoan. Each will be explained in their part.
I guess by rule of thumb Paramecia should be first thanks to the protagonist having one. According to the creator of One Piece, Paramecia-type devil fruit allow the one who ate it to use their body as an extension for the abilities. In the case of Straw Hat Luffy, he can stretch his body and skeletal structure like rubber. Yes, it does not have any effect on any non-solid matter. Most treat him as a person who bounces back immediately and think the devil fruit is the reason. However, he has always had this mentality. Even to the point of scarring himself at a young age. The other Paramecia users also have a similar outlook in relation to their devil fruit. Each one ate the fruit already having a personality to match it. From Doflamingo being a puppet master to Buggy being a trickster.
The second group is the Logia. These are meant to transform the users own body matter into different elements or natural phenomenon. From what we've seen, each Logia has a different level of change on the users body. Among these are Flame Fist Ace, Kizaru, and Sabo. These are placed on the level of difference it made on the person. Sabo has had very little mental or even emotional change. Kizaru and the other Admirals aren't shown to have any side effects. However, Ace is actually shown having symptoms of heatstroke that may be tied to the constant use of his devil fruit abilities. He has gone comatose for hours at a time, and even while doing common things like eating. If this is true, then the overall strength of a Logia fruit could be tied to side effects on the body while not being used.
Last, the Zoan fruits are capable of transforming people into hybrid or full animal forms, and in one case the reverse. The overall nature of the animal is seen in the human while in hybrid form and full animal form, while some have been completely changed by the fruit mentally. Notable users of this fruit are Tony Tony Chopper, the Impel Down jailers, Marco Phoenix, and Momonosuke. The Impel Down jailers have what is known as awakened forms that show they lose all humanity and are basically on the level of the animal their fruit possess. Marco Phoenix and Momonosuke are Mythical Zoan users, while Marco personifies his Phoenix form by being an attacker and healer for his men, Momonosuke has very little control of his fruit making it harder to pinpoint whether he is truly affected by the fruit itself or his own fear of the form the fruit transforms him into. Lastly, Tony Tony Chopper is the user of the Human Human Fruit. He has notably changed from basic animal urges as a deer into a fully-fledged doctor and even has acquired human speech while in what could be considered his natural form. Once again, across the board this group of devil fruit has various levels of change.
From the research done, I can confirm that devil fruits of the Logia and Zoan type can have major mental and physical side effects if used constantly. While the Paramecia can affect the body in different ways and have almost no mental side effects when being used other than overconfidence