Prenatal ultrasound cannot diagnose all problems with an unborn baby, so you should never interpret a normal prenatal ultrasound exam as a guarantee that you are going to have a normal baby.
Some abnormalities are very difficult to find, or may not have developed at the time in the pregnancy that you have your scan. The position of the baby in the uterus has a great deal to do with how well we see the heart and spine, for example. We will sometimes ask you to return in a day or two to complete the examination, in the hopes the baby has moved.
Images tend to be dramatically clear in thin patients with lots of amniotic fluid (fluid is the sonographers friend), and frustratingly fuzzy in heavy women, particularly if there is not much amniotic fluid.
As in almost every endeavor, there is a wide difference in the skill, training, talent, and interest of both prenatal sonographers (the people who do the scanning) and sonologists (the physicians who interpret the scan and write the reports). The general level of skill and training in prenatal sonography is constantly improving.