Well... I'd have to say Death Knights have been nerfed several times.
Still, in relation to the rest of the field, they're one of the stronger classes.
I'm not sure if you can go with 5x DK's and no heals.

Gearing is always an issue as well.
5x Elemental Shammies can destroy heroics once their geared a bit.
When they're starting, there's no chance to complete one with a tank type.
With a bit of gear on your team, you'll kill that much faster and live longer.

I cannot say if the 5x DK team is alive or dead.
For pvp's, especially world/battleground, they're powerful especially against casters.
Arena likely not so much.. they're as good as most 5-box / 1 class combo.