That sort of tatic, "swapping" would probably work once or twice against an, at least mildly, smart player. So while its cheap, that sort of thing happens and you just need to learn how to counter it. Now if something has no counter in the game mechanics, then thats a different story. But becuase some one got a bigger army faster and used them to decimate their enemy quickly with few or no causalties, one would call them cheaters, then there are 'cheaters' in real wars as well. However if the same player managed to repeatedly decimate the oppenent with the same tatic, then there may be a balancing issue. It would really need to be determined on a case by case basis and either solved in an open strategy or fixed in a patch.
On repeated occasions, in the demo my falcon wiped out the entire army with no back up at all. Thats the sort of thing that needs to be fixed. What if one team has a hero and the other does not? Tough luck, get a hero, but there should still be ways to remove a hero from play so the heroless player does not get totally decimated based on that small fact.
Balancing while being different is very important, but difficult. Look at SW:GB, it was balanced, but mostly because all the armies where almost identical. So i hope this will be balanced while having very different play styles.