The classic Mad Scientist is a Devisor. The devisor can create anything as long as they have a coherent theory of how it works. That theory doesn't have to correspond to the actual Laws of Physics, and in fact it almost never does. In some ways, the Devisor trait can be compared to the magical Law of Definition. This process is called the "Schimmelhorn Effect," after a character in a science fiction series named Papa Schimmelhorn. As Adam Savage says, "I reject your reality, and substitute my own."
As a consequence, devises can't usually be replicated, even by another devisor, and they may not work for anyone else. Murphy's Law, as applied to devisors, adds explosions. Worse, an angry devisor can pull off almost anything, especially aimed at the target of their anger. But that doesn’t mean that it will do the same thing when used on someone in cold blood.
The fact that they're not replicable is the basis for one of the common jokes about the difference between Gadgeteers and Devisors: "Gadgeteers can go to the Patent office, Devisors can go anywhere but the patent office."
Devisors are ranked on a 1 to 7 scale, however that scale hasn't been published. One can presumably make the usual assumptions about it, such as the higher, the fewer, and that higher levels can do cooler things that are more likely to work right and are more likely to be usable by others.
Devisors appear to be unusually subject to Diedrick's Syndrome, commonly called "Dricking out". The two most obvious cases are Mega-Death and Belphegor, although Mega-Death may also be a victim of tampering by Don Sebastiano.
The Gadgeteer and Devisor combination is rare, but there are several examples, such as Bunny Cormick. Some of them can tell whether their creation is a gadget or devise, some can't. Bunny is one of the ones who can't. The Advanced Technologies Program has procedures for vetting whether a devisor's creation is actually replicatable using the usual Laws of Science, and hence can be patented, the infamous Devise Test.
Likewise, the combination of Mage and Devisor is rare, Nephandus being one example. Apparently Nephandus, at least, has never been able to integrate the two in a manner he considers satisfactory.