What are the short-term and long-term risks of cheekbone reduction?

There are complex bony cheekbones to reduce the cheekbones.
You may also want to consider augmenting the areas around bony prominences with autologous fat-fascial grafts, especially if your face is very thin.

For the most part, this is not commonly performed in the USA except by physicians generally experienced in craniofacial surgery.
The risks short and long term are primarily related to nerve injury and secondarily to the sinus and bone healing complications with symmetry being difficult to guarantee.

Cheek bone reduction can be done by shaving the bone as well as performing osteotomies on the bone to change the angle. This is not a common procedure in the US.