I can't be sure that cinnamon is beneficial, though my results could be considered unusual.
My Hba1c after 6 months is 41, and it was 91 at diagnosis as a type two diabetic. I have a large mug of coffee with cream once or twice a day - about 10 or 11 in a week. I add a small pinch of cinnamon and a tiny amount of salt in this hot weather.
I do avoid all high carb foods but eat a wide variety of low carb ones, 10 percent carbs or less.
I used to work where spices and herbs were freely available from samples taken for testing, and as I am being a little selfish these days and eating the foods I find most acceptable I bought cinnamon for my coffee - it is ground coffee and I drink it with cream.
I stopped all medication after 5 weeks as I felt so ill on Metformin and Atorvastatin. I rely on diet alone. It is not a problem to add cinnamon into my diet and I have not noticed any disadvantage.
I have suspected that I have been diabetic for a long time, since an extended period of antibiotics, but I was eating according to the Atkins diet to control my weight, and I had access to the spices during that time and for quite some time after I left the post - I kept a large amount of them in the freezer.