Always check the individual equipment types that compose your units. For example, an arty unit with a range of two might have 1 piece of 122mm gun with a range of two, but have 49 pieces of 76mm guns with a range of one hex. So every time you use this unit at a distance of two hexes, only 2% of the unit is actually contributing to the combat! This is especially telling in fleets, where the two range is generally the single battleship, and everything else (the destroyers, cruisers, etc, which are considerably more numerous) will not participate in a 2 range combat.
The effects on air are devastating in 'mixed' equipment type air units. As an exmaple, in 1940 a WWII F/B unit is composed of 30/30 Battle (range 27 hexes), 32/32 Defiant (range 13 hexes), and 0/24 Tempest (range 41 hexes). The Tempest aircraft will not be produced until 1944, many turns from now. But back in 1940, the air unit counter still lists a combat range of 41 hexes. If this unit was involved in a combat at a range of 28 hexes and beyond, it would instantly evaporate, as it would have no equipment that could particiapte in the combat. I have seen close to a dozen aircraft untis evaporate in a single turn when caught in this 'kill zone', so watch carefully, and plan accordingly!