The whole issue is cleave vs single target damage.

Assuming you don't close to melee, they either charge you or you exchange spells at range.

The felguards will close the distance either way.
Which means you have to deal with their cleave type spinning axe attack on your team.
DPSing the pet is probably not a good idea, as they will be healed as the warlocks DPS you down.
A felguard team (demo spec) will not have Shadowfury to stun you.
I would take 5x Hurricane + 5x Starfall over 5x Rain of Fire, but the Felguards will interrupt you some.

If you are in melee range, the warlocks have a huge edge.
Add in Immolation Aura and Hellfire to the mix.
Shammies could add 5x Fire Elementals and a resistance totem.

The big drawback to Hellfire is that it hurts the Warlocks too.

Anyway, my kitties did not like the mean warlocks.