Nuru is incredulous when she sees Requiem for the first time, but squares up immediately when told "It is a bringer of ruin". It seems like a jarring quick turnaround until you remember Nuru is from Starbound and "Ruin" is a VERY loaded term for her.
Valder is the only commander whose groove has the Very Fast recharge speed, meaning he is fully charged from attacking and killing a single enemy, his groove creates an undead soldier which is a lot easier when you just made a fresh corpse.
The various conditions for the units' critical hits make perfect sense considering their equipment, role and background.
Swordmen crit when they're near their commanders, but they generally don't seem to be described as well-eqipped or well-trained. (Except the "elite" Heavensong Lionblades who may very well be hindered by their equipment) Being near the person giving them orders when they're still rookies would be very emboldening and encouraging them to perform better. In Felenheim's case, they might just be more afraid of their commanders than their enemies.
Spearmen who crit when next to another spearman are better-armoured and stronger than the Swordsmen. They even crouch to make themselves a harder target. Much like the Spartans, the adjecent Spearmen could cover the flanks of their attacking brethren so they can strike without fear.
Dogs fighting in packs is nothing new. They litearly hound down enemies by critting when their target is already next to another allied dog.
Rangers crit if they don't move in the same turn they attack. Makes sense, they have more time to aim.
Mages crit when in a spot that offers them 3 or more defense. A well-defended mage is a relaxed mage, and can focus more mental power on channeling their lightning.
Knights crit if they move 6 tiles away brefore attacking. The distance helps them build up the speed they need to deliver a most devastating strike.
Ballistas crit when the target is at minimum range (2) while Trebuchets crit at maximum range (5 and 6 if weather allows). Ballista bolts are launched straight forward and don't lose as much speed as they travel, while the trebuchets lob their ammo upwards. The further it lands, the higher it has to be flung, and the harder gravity brings it down on the enemy.
Giants get crits when at low health. As you knock armour off of them, it becomes easier for them to wind up a punch and wallop you upside the head.
Aeronauts crit when they're flying over mountains. This is the Cherrystone Harpy's natural habitat, similar to Heavensong Tengus who hail from a mountainous region. Felheim Vampires are a "historically isolated race" who only recently joined the war, so it makes sense they'd enjoy the quiet desolation in mountains, where many units can't even go to. I haven't figured out the Leafwings' justification for it yet, though...
A mobility advantage. Winged aeronauts can use the peaks and cliffs to full effect, whether it be staying out of a melee unit's reach or going low to find an angle on an airborne aggressor. Wings and legs give them better options for using the rough terrain than hovering Skyriders.
Skyriders use lightning, and crit when the target isn't next to an allied Skyrider. Even when critting, they can't one-shot an enemy Skyrider unit. Perhaps their rides partially function as lightning rods...
Dragons crit when their target is standing on a road, where there's nothing to protect them from the fire. You might think that they'd crit against someone standing in plains where there's grass, but plains don't have to be even and the terrain irreguralities or simply being able to dive in the grass could make you a harder target for a dragon - you don't get such a comfort on a flat, open, empty road.
Amphibian crit when in water, not shocking. They're attacking from their natural habitat, or in the case of Heavensong Kappas, simply because that's what their equipment has been designed for. Likewise, turtles crit in deep water, where they can move freely. Their attack is even a mini-tsunami, which shallow waters are able to partially absorb with the land beneath them.
The Harpoon ships and Warships, I couldn't figure out why (crits from reefs and beaches respectively), but the crits complement their respective defensive and offensive roles.
In the case of Warships, beaching themselves may give them enhanced power by eliminating the rock and sway of the ocean, allowing their cannons to fire at full-power and hit their target squarely. As for harpoon ships, harpoons are generally used for hunting at-sea... and there's nothing quite like a reef to hide in before springing your harpoons on your unsuspecting quarry.
Snipers deal extra damage on the last bullet they have before needing to reload. It's all the more important to make that last shot count, after all.
Commanders don't crit at all, but they don't need to. They got their grooves.
Koji's arcade ending. Sure, Requiem doesn't corrupt him now, but unlike other endings that shrug it off, it's not destroyed, either. No, the Artifact of Doom is now safe inside the armor of a walking weapon of mass destruction, in an ideal position to slowly corrupt its pilot or seek out someone who can be corrupted. The joke is that Children Are Innocent, but Koji won't be a child forever.