First, we should bear in mind that each unit exerts a ZOC into the six hexes surrounding it. A friendly unit negates enemy ZOC, which can be crucial to rescue surrounded armies or corps for example. After entering into a ZOC the unit must stop immediately and cannot continue movement. when starting a new turn, the unit may be moved out of the ZOC without paying additional cost.
Second, the stacking is a feature present in SC but it was absent in USE. Here in SC the stacking limit is six total corps. An Army or HQ corresponds to 3 corps. Each Corps or Fort is 1 Corp. A maximum of two naval units may also occupy a single hex. Flak (yes it includes this nice units!) do not count for stacking purposes. Only two air units can trace range from a single Air Base.
Once a target hex has been determined, all adjacent units can contribute to the attacking force by summing the strength points. Note that the target hexes and attacks have to be declared in advanced. The attackers may have flank support bonus and HQ support bonus.The latter is determine depending on the distance from the HQ and the attacking hex. The difference between the attackers strength and the defenders strength determines the column to use on the Combat Resolution Table (CRT).
As we can see here, we have stacking, simultaneous attacks along a front by announcing target hexes, and independent movement and combat phases. These features are different to those of USE. Each player may have its favorites. I think most of us are familiar with these features and USE mechanics are quite new to us. However, it is good to have variety and explore new concepts such as the ones introduced in USE. It is like comparing apples or oranges, both are good depending on what you want at that moment.