So I've been hearing that stacks have been overpowered, and saw some of that myself, but I noticed something, and did some reloading to test it out, and discovered why that is.  Frogboy commented in this thread https://forums.elementalgame.com/392412/page/1/#replies that a stack of units doesn't roll for their total attack, but instead it's treated as a group of individual units.  Except there's a major loophole to that:

Apparently the units in a stack are acting out the movie http://en.wikipedia.org/wiki/The_One_%28film%29  Whenever a stack takes damage, some of the units in that stack will die, however, in a tactical battle, the unit's attack and defense doesn't get reduced when a unit dies (I think it does after the battle ends, but not within the battle itself).  That means that the attack and defense gets split between fewer and fewer units, until you're eventually left with a single demigod of a unit that possesses the attack and defense that the entire stack had.

I tried this out reloading an autosave several times to test this out, when my single archers were shooting a stack of units that was full, they were able to hit it and do damage most of the time.  However, when that stack got damaged, and lost units, it's stats remained the same, but it had fewer units, and now my archers had a harder time hitting it.  Eventually, when it got down to 1 unit, it was extremely difficult for anyone to hit it at all.  But I still wanted a bit more evidence, so I went and reloaded and moved my stack of archers into the middle of the enemy forces.  They got hurt a lot at first, but as their numbers dwindled, it was eventually left with a single unit that they had a hard time doing any damage to.  I then took that unit, an archer with a 6 strength bow, just 1 of them left, and shot the other stack of units for 19 points of damage.

Since the attack and defense of a stack of units doesn't decrease during a tactical battle, losses in units actually make a stack become much stronger, rather than weaker.  So instead of a single unit left of a stack being equivalent to an individual unit of that type, it instead becomes a massively powerful unit with the combined attack and defense of the entire stack