McCree and Zarya seem to be performing a bit better than expected and Blizzard are testing significant but fair nerfs to both of them.
McCree can be annoying with aggressive rolling into flashbang which seems to be the main reason the Blizzard balance team is messing with his numbers. The Experimental card currently has a reduction on his roll distance of 20 per cent and a nerf to reload speed, which went from 1.2 to 1.5 seconds.
In other words, they are trying to give the cowboy fewer opportunities to roll aggressively to deliver that flashbang and then unload on helpless targets. The roll nerf will obviously shorten the distance he can cover so he will have to risk more in order to walk up to the target while the reload nerf will push him to use the roll for getting ammo back more often.
As for Zarya, she's not getting nerfed that much when it comes to her total destructive potential. Instead, the nerf will punish her for not placing bubbles well.
Her primary fire will deal from 75 to 170 damage, depending on the charge, down from 95-170 per second. With no charge, she will now require 54 "rounds" from her gun in order to kill a 200 HP target, making it a 2.67s TTK. Previously it was 43 rounds, with a TTK of 2.15 seconds.
Overall, Zarya should perform noticeably worse when she's not protecting the right targets with her bubble, making the hero less forgiving and less oppressive. That said, if you are playing Zarya well and keep the charge up reliably, you shouldn't see any major drops in your damage output.