Speaking as a Rein main since S2, I feel double-barrier less the problem in and of itself, but rather the newer barrier tanks being able to do more from a distance while deploying them… while Dive has been weakened by increased CC and burst damage power creep, both of which are easier to apply to enemies diving your team than to coordinate diving the enemy.
Rein is kind of the gold standard for a barrier tank. It is strong, but comes with exploitable weaknesses. He can’t recharge it while it is deployed, nor can he attack while it is deployed. And there’s a delay before it starts to recharge that puts a limit to his ability to work around that weakness by toggling.
Sigma’s Grasp and Diva’s matrix likewise have this sort of weakness. They are very strong abilities, but leave openings. Neither can attack while using them, and Sigma can’t redeploy his barrier. So he opens himself up to things like Shatter, or being dove by Winston while helpless. And Sigma’s barrier is, like Rein, dependent on recharge.
Winston has a barrier on cooldown, but has short range and can’t adjust the barrier on the fly. And he doesn’t have a lot else he can do without closing.
Orisa, meanwhile, is the biggest offender in this regard. She has full range to her damage, her barrier is constrained by only by cooldown and thus cannot be punished with slow steady damage to create an opening (as with Rein, where once that barrier is down you have a window to push). And she can use her entire kit without constraint. It creates a ugly balancing tightrope where either the barrier or rest of her kit must be kept handicapped, lest she become oppressive.
Sigma is also capable of dealing impressive damage while barrier is out, but he does have a finite range that makes him vulnerable to ranged comps and airborne enemies.
In general, though, these are less of an issue than the massive powercreep in burst damage and healing, making it easy to burst down dive when they initiate, as well as burst heal their damage. The traditional counter to double-barrier was always Dive. When it was just Orisa-Rein, Tracers and Genjis were very good ways to deal with them.
Btw, I don’t want Rein’s armor/damage buff for that reason. Power creep needs to be dealt with anyways, and he’ll be stronger on his own once they are. This just adds to power creep and means he’ll be broken if powercreep is addressed elsewhere.
