As someone whose third-most-played hero is McCree, D.Va and Winston are not scary to me at all. In fact, when a team went six Winston on us the other day, we switched to three McCree and a Bastion to deal with them. McCree demolishes tanks and is probably the best short-range counter to Winston. Like, better than Reaper.
And as someone whose most-played hero is Genji, a decent McCree has me beat almost always unless I'm pinging from range.
Also, everyone complains about the stun-fan combo. But McCree would be fine if that were all he has. His real power is his left click at range, which bad McCrees don't seem to use well. This is why Pharah isn't great against McCree either; if she's in the air, you can drop her in two or three shots.