... won't you run out of enough military units to even form an effective fighting force against the monsters? Seems like its almost way to easy to destroy bases - but even that wouldn't be a problem except for each base stomped means you remove a military unit from play!
That means within 3-4 turns if all the monsters are stomping away, you could already be down 3-4 units PER army card, and you only have about 8 on each card.
Anyway, overpowered, or again, would you consider this a playstyle that is fair?
It's fair. The bases are so spread out and if you actually attack with military units, they get beat up and then go back into the supply. It takes several turns to even get most of the units on to the board if you are just deploying them and not fighting or drawing military cards.
I don't believe that they are meant to be particularly effective against the monsters, just slow them down and harass them a lot.