We lost our 11 yr old orange tabby, Pixel, about a month ago. He had an experience with Apoquel. So I thought it would be good to put another data point, if a confusing one, on your map.
Pixel was a rescue cat from an agency that initially didn't tell us that he had been returned, probably for his allergies, which manifested as itchy patches of redness, often with a brownish crust. By dietary trials, we determined he was allergic to kibbles, rice, wheat, chicken, turkey, beef, and pork. Pixel was always finding a way to get into food he was allergic to. After a bunch of times trying steroids, which mostly worked to get him back on an even keel, he started having more lesions despite sticking to a special diet. Our vet told us about Apoquel and that it had been a miracle drug for some dogs. We tried it and the effect was like magic. The skin lesions were gone in days.
After 5-6 months on the Apoquel, we noticed a lessening of Pixel's appetite and had to take him off of it. Appetite recovered some but not completely. We later learned he didn't want to eat due to bile duct inflammation/ blockage. Steroids and Amox/clav were given to try to reduce this, without much hope, but he recovered quite well. We put him back on half as much Apoquel, but the same appetite suppressant effect occurred, so we discontinued it. He lived for another year and a half with no bile duct symptoms but it was probably silently getting worse. Eventually he developed bile stasis. When the time came, we took him in for euthanasia.
In summary, the Apoquel worked great on the EGC (eosinophilic granulomas complex), but may or may not have had side effects that caused or worsened GI (his gastrointestinal) complications.