I am considering HIFU to tighten saggy jowls, are there any risks or side effects I should be aware of?

My main concern with non-surgical options for jowls is that they do not address the true anatomical reason for the jowls to have occurred in the first place which is a descent of the soft tissues and fat compartments in the face along with a weakening of the muscles. The use of a non-surgical energy based treatment  will not relocate the tissues back to their youthful position and will only provide a shortlived temporary minor improvement. The best procedure is a lower face and necklift which will relocate the tissues to their normal position and provide a longlasting effect. Of course facelift and necklift procedures is not one all-encompassing procedure since every face is different and the specific type of surgery and adjustment to the deep tissues ( the SMAS layer and platysma) will be different. It is best to seek out a surgeon who is experienced in all types of facelift surgery and who is able to individualise your surgery to the specific ageing features in your face. Beware of surgeons promoting one type of branded facelift or enticements into a local anaesthetic procedure. The ageing face needs an overall assessment and may require multiple techniques during the facelift to provide the best longlasting natural result.     

Jowls are best treated with a facelift type procedure.  HIFU will result in very limited results.  Considering the cost of such treatments, you would be best served with a surgical approach.