A rather unknown aspect of Supreme Commander 2 are unit costs. Depending on which factory is producing the unit, it will be built at different speeds, and even costs.
Factories, like all units, gain experience. Every time the factory builds a unit, it gains a bit of experience, and once it has reached the next level, units will cost less, and build faster. All the way up to level 5, where units cost 50%, and build 100% faster. However, a factory will always consume the same amount of mass per second, veterancy changes only the number of units you get out of the factory for a given amount of time and mass. This means that one has to balance production of new factories with utilization of existing factories in order to get optimal build rates. One tactic to lower the enemies production efficiently is to raid the oldest factories, which would have the highest veterancy levels. Even though he can rebuild the factory for a reduced cost, he looses the veterancy of the factory.
The UEF has 3 production experimental units: The Atlantis II Experimental Aircraft Carrier, the Mega Fortress Experimental Mobile Factory and the Noah Unit Cannon Experimental Unit Cannon. These units will build at a mere 1/3rd the cost of standard factories. This means it is actually quite smart to build these units, if only to use them as a factory. Furthermore, the Mega Fortress builds 3 times as fast as a normal factory, and the Noah Unit Cannon at 7 times the speed of a normal factory. This means that these units are actually cheaper than factories, in sheer production power, not to mention for a fraction of the production time.