Sooty mold is a black fungus that is very ugly. If too much sooty mold gets on the leaves it may interfere with the plant's photosynthesis process. Another concern is that the saliva of hard scales, which the insect injects into the plant while eating, is toxic to plants. For virtually every fruit tree and decorative plant there is a type of scale that will attack it. However, some plants are much more prone to it.
Scale is a major concern for citrus growers.
It is a fact of life that if you have Euonymus plants they will have armored scale. The only question is how severe the infestation will be. However, as Euonymus are a favorite food of deer, this will not be a problem for most you. If you plant Euonymus the deer will destroy it long before the scales do. It is difficult to treat for scale.