Ok. What did we get in XCOM 2 vanilla? Grenadiers. How OP were they? Let's see: Ability to use two explosives or heavy weapons in a single turn, check; ability to shred armor and obliterate entire groups of enemies, check; heavy armor and resistance to explosive damage making them very tough to kill, check; and ability to just be better at carrying 'nades than any other soldier, check. Grenadiers were f***ing insane, and everyone knows it.
Here is the gist of the "hero" classes we are getting: "Psionic wildlings" who need to gain power mid-battle to use their really good abilities, so they likely need to be babysat to not die. Reapers, soldiers really good at cloaking and sniping and sabotaging, but how effective do you think they're going to be once they're actually found out and can't rely on ambushes anymore? Not very. They snipe good, but when you're chewing on Heavy MECs and Vipers at point blank range, all the stealth in the world's not going to save you. That seems to be where Skirmishers come in, being the guys who use multiple-action abilities to handle several enemies at once. But if you shoot three times at a Muton with 50 defense in heavy cover and still miss, those actions wouldn't help you anyway. And if they get poisoned or Mindspun by a Sectoid, you're in trouble. If Mind Controlled, you're in even worse trouble.
What we have here are units with unique abilities for a different kind of dynamic. But they are very limited in number, and the enemy also gets new units and "heroes" to send at you, too. Yes, these hero units have abilities that may prove extremely useful for the player, but will they be a do-everything solution? It doesn't seem so to me. Grenadiers already fulfill that in spades by having huge amounts of explosives that can inflict critical hits, heavy guns that shred armor and do insane damage, Suppression to lock any enemy down (except Codexes and non cover-using units), ability to carry Flashbangs to nullify enemy abilities and other status effect-causing grenades, great survivability... I defy you to name one situation where a full squad of properly leveled and equipped Grenadiers couldn't just stomp everything on the map. They wouldn't even need healing because everything would be dead before it gets a chance to blink. They would stroll down the block, blow up the block, mow down EVERYONE, and walk off into the sunset drinking Mountain Dew. 3 pods activate, they eat nade-nade-nade-nade-rocket-rocket-saturation fire-suppression-rocket and are dead-dead-dead-dead-dead dead dead ded da-ded-dead-ded-DEAAAAAD-DEAD. Because having armor shredding, cover nullification, massive aim penalty abilities and huge AoE's on the same character is just OP. But no one talks about that, nooooo. Pff.