Is it "overkill"? Or is it merely just using mechanics to reflect a narrative idea, and not using mechanics to play out the board game?
An 88 hit point ogre could be deadly for 3rd level PCs, and an 88 hit point ogre could be a cakewalk for 17th level PCs. That is true. But even that 88 hit point ogre might require three or four 17th level PCs to hit it in order to kill it (PLEASE NOTE: I have not actually checked the numbers for my example so let's not get pedantic about what "17th level" characters can and cannot do-- insert whatever character level applies).
I echo and applaud your "please note" here; this is all for example purposes only.
Which is fine, it and of itself... but is there any reason "in-fiction" why a PC one-shotting said ogre would be a bad thing? If it's statistically and mechanically impossible for that one Fighter PC in the group to actually do enough damage to insta-kill an ogre and so that thing never happens "in-fiction"... again, that's fine, but why is that the only acceptable "in-fiction" result? The Fighter has to hit every ogre multiples times to kill it?
Using the minions rules is just one of the ways that allow for the "in-fiction" result of a Fighter taking an ogre's head off with one swing. Is that a problem "in-fiction"? Is there a reason why that shouldn't be allowed in the story of the campaign for the Fighter to have that heroic moment? I personally don't see any reason why it can't be allowed for the narrative of the story we are playing that the Fighter one-shots an ogre, but maybe others do?
Isn't that what crits are for? Your high-end Fighter might not one-shot every ogre she meets but now and then she'll behead one in a single blow, courtesy of a crit.
I will say though, that I suspect those that do have an issue with it, have the issue because they are looking at the combat from the "behind the curtain" board game perspective, and not from "within the fiction". "Behind the curtain" the player knows the mechanical board game rule of "Ogres are supposed to all have 88 hit points". And if you change that... you are change the rules of the board game. The fiction doesn't matter at that point... only the board game rules matter. And (general) you can't play the combat board game mini-game "tactically" and "effectively" if the rules of the pieces can change. If your Fighter can only do a maximum of 75 HP of damage with a swing, one can't have a "ogre" piece fall down dead on that one swing, because that breaks the "rules" of the board game (using the so-called "fiction" as the explanation as to why this "rule" of the board game is in place and cannot change.) The "fiction" is our "reason" why every ogre has to have 88 hit points in the board game (all our ogres are "healthy" and none are "invalid" or "children") and it's important to maintain that "fiction"... and yet it then precludes the possibility in the "fiction" of ever one-shotting an ogre-- for no discernable reason other than the "game rules".
To this well-put paragraph I can only say that just because our one ogre happens to have 88 hit points doesn't and shouldn't mean they all do; the same as not every 10th-level Cleric will have exactly 74 hit points (again, example only!) when fully rested. If however it is the case that the game dictates every ogre has exactly 88 hit points and every 10th-level Cleric has 74, then things have moved way further into the board-game realm than I want to go.
Also I guess we should keep in mind it's not always going to be top-end Fighters taking on our ogres; and a character (say, a Healing Cleric) that can only at best give out 20 damage on a hit probably shouldn't be one-shotting anything bigger than a house cat. And yet the minion rules allow this: by RAW that Healing Cleric can one-shot a minion Frost Giant, provided she can hit it at all.
This is how many folks wish to play the game, and that's cool. I don't have an issue with that. You do you. But I just do not happen to fall into that category of person. I find the narrative results of the entire fight to be more interesting than the board game that has to play out to achieve said narrative. The board game can be fun... but I don't play D&D for the board game, I play for the story that comes out of it. And if that means mechanics change to create new ways of generating narrative... no problem with me whatsoever! After all... isn't that what Swarm rules are? Changing the mechanics of standard board game pieces to achieve a different narrative result? Or Mass Combat rules? Pulling so far out of the board game that we institute a whole new set of board game rules that we plop in and play out to emulate a different story in our game? If most of us have no issues using new and different mechanics to emulate those narratives... using minion rules to emulate a new narrative shouldn't be much of an issue either in my opinion.
Now here we diverge fairly sharply.
I too want the fights to be interesting, but am not as concerned about the "narrative" side of things; that comes out in hindsight. I'm also not after the whole "cinematic" style; if cinematic moments happen, they happen, but trying to force them (or make them too frequent) does them a disservice.
That said, one of the key elements for me that makes the stroy tie together is internal consistency within the fiction. Changing the same creature's mechanics based on who-what it's interacting with breaks that consistency wide open.
I don't see swarm mechanics as inconsistent, however, provided they're simply the aggregate of the individuals involved. Thus if each Orc averages 10 hit points and there's twenty of them in the swarm, then the swarm should have 200 hit points. If each Orc has a 10% chance of hitting the foe then that's reflected in the swarm's average damage. And so on. That said, where swarm mechanics fall apart for me with anything bigger than cats is that they tend to ignore the fact that only so many individuals can attack a single person at once due to reach/spacing issues, and instead assume the whole swarm is attacking.
