Me, @crispymorgan @Diddums and @Greboth gave this a right good go last night.
Hard mode completed on both specific and all heroes modes. Intense and well worth doing.
There's still Expert and Legendary difficulty. This is bloody tough. As soon as one wipes it can be over in seconds. Providing the one dying isn't Mercy you could be okay. Healer goes down and 95% of the time you're screwed.
You can get overwhelmed by enemies when waiting for the payload bomb to get going. The Bastion wrecks you quickly if you haven't got cover or Reinhardt's shield cover.
Many of us on here I think could complete Expert with a flawless run. Legendary....yeah, that's another league altogether.