There are several disadvantages to using ultrasound in medicine, one of which is the potential for operator error. Some machines also produce images with limited resolution.
Ultrasound cannot be used to study any part of the body containing gas, so it is not a useful tool for diagnosing problems of the lung and digestive tract. This type of study is also very limited when used to identify fractures and other problems with the bones.
The CancerQuest website says some ultrasound scans may produce false positives, causing patients to undergo biopsies and other invasive procedures that are not necessary. Ultrasound is also unable to detect calcification of the breast tissue, making it less useful for detecting some forms of breast cancer.