It should be a very safe combo and very helpful on teams, but be prepared for slow soloing due to the low damage output.
An Earth/Dark controller would have a lot of advantages as a team character, and would even be an extremely safe soloing character.
Stacking Stalagmites and the stun from Howling Twilight is a nice thought that comes to mind, and I don't think anyone would complain about stacking Quicksand and Tar Patch would be a nice way to debuff. Add in Darkest Night, Volcanic Gasses, and Earthquake with an Achilles Heel proc, and you've got a beautiful chaotic tower defense.
Rocky will even make it easier for you to wander into melee and make good use of Soul Absorption.
No matter what, you'll be feeling the low damage. I'd say that Epic/Patron attacks will be a must (I would personally pick Ice, Fire, or Earth Mastery depending on whether you want a varied, offense-based, or more melee-based playstyle). The question is whether it's enough of a problem to not enjoy what the character CAN offer.
Too many alts to list.
I have kicking around in my head a concept for Earth/dark affinity. Any experienced users have comments on the proposed combo?
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.