Question, is it currently viable to place an Airport or Factory, and then use mercenary cards or other discount unit cards to place units within the starting borders of the Airport/Factory?

It seems to me like the Airports and Factories are kind of underplayed, I have only ever seen 1, and that was in a sealed game, and if you could use it in tandem with cards that enlist units 'in the starting area' but instead place them at the Airport/Factory, well I think it opens up a few more possibilities, and would make them more powerful.

Also, If there is not already an infantry equivalent, then there should be.

As far as being underplayed, here's why I dont:

Airport has 75hp, and unless you already are close to deciding your win, will likely be destroyed in 2 or 3 turns
Tank factory is high on my wanted list..

I would say that you cant put your enlisted or mercenary units around an airport or a factory since you only can put them in you starting area and that are the grey fields around your HQ.
And they are underplayed since not everybody owns them and those who do probably have better cards to use

Using mercenarys with factory would be too powerful. You just need to get to enemy base, pu tfactory there, then put 6 mercenaries there. ( you could do that, casue they are really cheap. Enemy would have no chance.

But anyway you can do something like that with 2 airlifts

Tank factory is a beast of a card. For 100 gold you can deploy any ground unit but infantry from it. I just wish i had one:(