Lack of chill makes buds slow to break in spring or not at all. This may mean that the cross pollinating variety's flowers are over or not yet in bloom as different varieties have different requirements. Also fruit quality and maturity will be non-uniform on the tree, and harvesting and storage will be difficult. More is known now and a number of models have been developed to more accurately calculate winter chill units. Very low freezing temperatures are thought to not count.
Most varieties require a period of heat afterwards to complete the bud break.
Ashcroft et al. in 1977, published a method determining chill units and growing degree hours from temperature data and observed dates of full bloom. In the last decade or so, moves have pushed ahead to grow these fruit in warmer climes of Australia, that do not have sufficient winter chill any winter, such as around here on the North Coast. Other areas have enough chill most winters but others are too mild, such as at Stanthorpe in Queensland, parts of Southern Victoria and Western Australia. How much winter chill is required for a variety or species is often decided by where that plant grew originally. Varieties grown in high latitudes and cold climates are likely to require high chill.