I just don't feel the sentiment that stacks are overpowered & ridiculous. They only "seem" ridiculous because the AI doesn't effectively counter them. Stacks are capped at 12, and most units operate singly (summoned monsters in particular). Eventually, summoned monsters become obsolete as squad & warfare tech increases, but they're effective early in the game (like the first 100 turns). I wouldn't mind seeing some kind of leadership trait though that defines a max stack size (instead of the arbitrary 12). Some non-adventure leaders should be more limited as to maximum stack size (say half of 12), only champions with very good leadership would get the full 12 units per stack. Units without leaders, say, are limited to 4 per stack.
.. something like that anyway.
Of course, the AI should arbitrarily be given an increased stack size, say by 3 on "ridiculous" setting, 2 on "hard", one on "average" none on easy.
Most TB wargames usually have some kind of command & control mechanism that defines how many units a general type leader can influence. In this game, it would be very easy to have a "leadership" trait that can leveled up just like any other trait but defines how many units can be included in the leader's stack. Only a couple of very strong units would could have very large stacks (just like there's only a couple of 5 or 4 star generals in real life).
But, I don't think you're talking about "stacks" as much as "squad" size. The attacks & defense of "squads" are just added together. i.e. a 4-man squad with attack 16 defence 20 and 24 hitpoints really has 4 individual guys each with 4 atk, 5 def, and 6 hp. From what I understand, the system just calculates 4 seperate attacks (each of the four guys) and adds the end result. In other words, it's not multiplicative but additive. The end result is that you can cram more guys into a "stack" (not squad). So, instead of 12 single guys capped (as it is before you get 4man squad tech) you can have 48 guys capped (12 groups of 4-men), and later 8 man squads once that tech is unlocked (12 groups of 8 or 96 men in a "stack").
I can understand what you're saying with the spells, but you're realling paying a lot more for the squads than for singles (a well equiped 4-man archer team of mine costs 300 gold in addition to other resources & crystal). That's quite expensive, so I would expect to see my money's worth on the battlefield .. this is like paying for a tiger tank instead of a sherman.