There are some good things that happen if our weather is mild for most of the winter. Flower buds that are more sensitive than vegetative buds to cold weather survive in much higher numbers. A good example is forsythia. In a very cold winter, flower buds are killed down to the snow line, if we have snow (we call that a snow skirt of bloom), otherwise clear down to the ground. In a mild winter, we get that great bloom show from the ground to the tip of every branch. Another sensitive plant is the peach tree. You can count on foliage every year, yet flower buds are killed and why a peach may only bear fruit every few years (a mild winter).
However, a downside is that a larger percentage of overwintering adult insects can survive during a mild winter since they are set up to withstand much colder temperatures without dying. An example of an overwintering adult are scale insects on just about any kind of shrub or tree. Recently magnolia scale has been on the increase. Scale insects affect the long-term health of landscape plants and the home orchard. Insects can overwinter as eggs, larvae, pupae too. Eggs are often found in the cracks and crevices of the bark and buildings, larvae and pupae in the soil just below the surface, or in the case of some moths, bagworms and butterflies, as a cocoon attached to a branch. Overwintering adults will collect in strong numbers in cracks and crevices. Lady bugs often will be found in the leaf litter at the base of shrubs.
Plant diseases are going to overwinter in a stage that are likely to survive no matter what kind of weather we have. What will make a difference is extended cool or cold, wet weather in the spring where leaf litter containing the fungal spores has a longer infection period. This is why those pesticide labels address the length of the spray schedule based on local weather patterns.
No matter what kind of a winter weather we end up with, gardeners know that diseases and insects will develop right along with our landscape plants, so be on the lookout early and often for potential problems.