My first team to do instances was a Prot Pally with 4x Shammies.
One of the shammies is dual spec for Resto when I want it.

This team has an easy time in instances.
The pally never loses aggro on single mobs, if the pally pulls.
And gets it fast, if the shammies pull with chain lightning for mobs outside of the pallies range.
And for AoE targets, the pally has passive threat from block, retribution aura etc.



My mixed team started to do instances.
Disc Priest healer at 75th, Feral bear at 76th and 70-72's Elemental Shaman, Affliction Warlock and at the time Frost Mage.
There were no issues on single target threat.
However, the druid had serious issues at AoE threat.
The priest could take aggro on secondary mobs from heals.
And the Shammy could using the click castsequence FS-LvB-CL-LB, which is CL 3-4 seconds into a pull and then every 3.5 seconds thereafter, unless recasting FS/LvB.
The pally at 71st was a much stronger tank for secondary targets when boxed, then the druid at 76th...

However, when I one-box the druid, I have no issues with threat for singles or groups.
It's just the pally is incredibly easy to box, for both single and aoe threat.
The warrior might be in this category - amazing one-boxed, but too many things to click on demand to be boxed very effectively for multiple target pulls; not sure on this, have never boxed a warrior.

Have not really tried the druid in instances since...
But have broken their moves down to three buttons, instead of a click sequence.
Which imitates the moves I most frequently use when one boxing.
And then have the dps with their click sequences on each of these keybinds.