There should be a limit on the number of spawned unit at any one time per producing unit (so say 8 fighter jets per carrier), with options on how to handle them when the limit is reached (ie, destroy oldest and spawn new one, dont spawn, etc..). Also, maybe those spawned units should blow up if the producing unit is destroyed.
And perhaps limit the range at which such spawned units can travel away from their producing units.
Those spawned units should be different then say real produced units (ie a Carrier that launches fighter jets shouldn't be considered the same as the Summoner unit that produces units from the magic faction).