The Union air power is awesome and the Union can basically blow the crap out of two Clan Warrens per turn once their fleet is concentrated, but since the air units can't move in Reaction or Exploitation movement they are slow. The combat process almost always costs the attacker some steps, even at high odds, since the combat is missile-based and losses represent munitions depletion, so even the best attacks make one pause to consider whether it is worth losing the steps. Rebel units are very weak on defense but strong on attack, which always leads to interesting combat situations. Similarly, air units never take losses when supporting, so a Union air unit or two allows even the cheesiest Mutant mobile unit to have a tough defense with no risk to the Union air units providing the strength. The Union should never keep air units in cities, since they don't suppress revolts and can be killed easily by a revolt.
Air units are quite good at defense in the field so they have nothing to fear by leaving the cities.
The Rebel really should just strive to keep his Clans alive until lots of revolts and uprisings occur, since the Union has the edge in combat power at the start.