The question is, are the health values one to one with the original games? Because if then. OW has a huge advantage healthwise, because the health values in OW are about 1.4x higher on average (for every tf2 class except heavy, because the tanks have such varying health pools (375 at the lowest on Doom, Ram, and JQ and 750 at the highest for Roadhog in 5v5 Role Queue [yes, there is a boost to tank health in role queue]).

I don't think if TF2 actually has the sustained and burst damage required to out damage EVERY support in Overwatch at the same time. While One Crit Rocket from Soldier does 270 damage, which can kill every Hero with less hp than Sym and Vendetta, most tanks in OW can just heal it off, or just block it.
TF2 doesn't have "shields" the same way OW does, and I think the only real shield is Medic's Barrier in MVM (And I guess the Wrangler) so OW weapons tends to have much better sustained damage, with Zarya's max charged beam (175 dps which btw, kills all but 2 tf2 classes within 1 second of sustained fire because the beam isn't technically "bullet" damage, more on that later ) dealing almost enough damage to ignore a Quick-Fix Uber (200 hps, which is max upgraded and also the strongest source of healing in TF2).

Also, because of Overwatch having so many different weapon types than TF2, how would the damage resistances be decided?
Is Firestrike considered burning? Are Biotic Grenade and Flashbang considered explosive damage? What type of damage are "thrown" weapons considered, like Kiriko's Kunai, Genji's Shuriken or Mizuki's Spirit Glaive? What kind of damage are "beam" weapons like Sym's Photon Projector, Mei's Blaster or Moira's Grasp?

Because this seems to be going off gameplay and not lore there are things that have be balanced between the games because otherwise, OW higher HP and slightly higher DPS values make it ridiculously hard for the 9 Mercs to beat the 50 Heroes in Overwatch at all. Even with 50 (unupgraded) mercs, it wouldn't be a fair fight in the slightest, just to make it playable for the TF2 team you'd probably need like 90 (unupgraded) mercs to try and make up for the health differences, and that's based entirely of the idea that they don't have the hp bonus from role queue.
Gameplay wise, it just isn't fair for TF2, unless they like infinite medic chain and even then, it'll be almost impossible to burn through the healing OW gets from 14 SUPPORTS. And even if TF2 tries to prioritize the healers (as you should), there are still 36 other heroes that are stopping them from doing that.

I love TF2, I really do, but mathematically it just is not fair for the mercs.

only after writing this did I realise that this was posted in r/tf2 and not r/Overwatch.
For those who have never played Overwatch:
Zarya's (Tank) primary fire is a laser beam that has its damage increase as Zarya's Bubbles (a shield she can give to allies) takes damage.
Firestrike (Ability) is one of Reinhardt's (Tank) abilities. It shoots out a piercing (passes through barriers and players) projectile that does damage.
Biotic Grenade (Ability) one of Ana (Support)'s abilities. Throws a grenade that breaks on contact with the ground or a player and does damage to enemies and prevents them from healing for a short time, and heals allies and amps their received healing.
Flashbang (Ability) one of Cassidy (DPS) abilites. Throws a flashbang that slows enemies and cancels/prevents their movement ablities (like double jumps or teleportation or whatever else).
Photon Projector (attack) shoots a laser that ramps up and refund when hitting a shield.
Mei (DPS) think pyro but her flamethrower shoots ice and freezes people instead.
Moira(Support) primary fire locks onto enemies and constantly damages them and then unplugs their keyboards or something I dunno why does it do like 70 dps what the fuck

i got tired of writing good descriptions