There are two different types of wishwings. The most common is negai type. These are wishwings born from various objects filled with positive wishes. The vast majority of wishwings are negai.
Negai types start off with a singular powerful wish, but after that wish is granted they lose a lot of strength and must rest for several days to recover their energy. After that their power is lessened and they can only grant small wishes related to the original desire that created them. As negai types grant more and more wishes they grow stronger and capable of granting more powerful wishes. If a negai type continues beyond this they eventually gain the ability to grant wishes outside of their spectrum. negai types that are long lived often tend to have great power and are highly regarded not just by humans, but other wishwings.
The second type of wishwing are shinigami types. Shinigami wishwings are very rare and only occur in drastic situations, often being war. When a massive amount of negative wishes are poured into an object and deadly being can emerge. A shinigami wishwing seems similar to a negai type at a quick glance, but there are a few differences. Shinigami wishwings have black schlera and often large clawed feet. Many Shinigami have weapon shards floating around their body they wield using their powers. Shinigami wishwings have no love or desire to help humans and a born to be killing machines. Often bringing destruction to both sides. However, once the war ends, shinigami wishwings tend to simply vanish. It's unknown whether they fade away once they use their energy, since it cannot be recovered due to lacking the ability to grant wishes; or if there is another method they use to keep alive. No shinigami wishwings have ever been spotted twice so there are countless theories, but none have been proven.
Although there are only two types of wishwings, there is actually a third type that is even rarer than a shinigami and so far has only ever happened once in history. When in times of war half the population wishes for peace and the other half wishes for death, a unique and burdened wishwing is born. One that is half negai and half shinigami. This type holds unbelievable power and possesses the ability to use the shinigami's weapon shards as well as grant wishes with talismans. But this half breed type suffers immensely having to fight between it's desire to help humans or kill them. Extremely little is known about this type as it requires a special scenario that almost never happens.