The biggest risk is failure to achieve a good result. Symmetry is tough. It's not easily reversed and once you do, you tend to look worse than you did before. It's also not permanent, as skeleton pulls back a few mms starting in your 40s. What looked good when you were younger often doesn't so much as you age. As the fat pads thing, the underlying bone structure and it's inconsistencies become more prominent. Other than that, I think cutting and moving bone may require the surgeon to manipulate the soft tissue around there more, compared to sliding in an implant. There's a lot of muscles in the face and a few important nerves in that area that you'd rather not disturb. I think this isn't too much of a problem in practice for an experienced surgeon. Failure to achieve good results is the biggest risk.
If you want cheekbone augmentation, then my advice is to find all the ways of fixing it. Then find a guy who does them all and ask him to recommend a technique for you. Then find a guy who's most experienced with that technique. Have some humility, you don't understand or appreciate all the issues involved so find a guy who does and can make that decision for you.