I tried looking up the installation instructions online, but I couldn't find any. Maybe Slavens or someone else who has it can confirm, but it looks to me like the brackets that hold the carrier get bolted on sandwiched between the tailgate hinge and the body. At least, that's how the Maximus 3 tire carrier mounts up which looks to be a very similar design. The body is reinforced there to take the weight of the tailgate and the stock spare, and Maximus claims it's plenty strong enough to take the weight of their carrier and up to a 40" tire (plus add ons like a high lift jack, rotopax, etc).
That sounds like it might be a bit much though... and I don't think I've seen pictures of one running anything larger than a 35". Either way, I think both the Maximus and Crawler Concepts carriers would be about as sturdy as most bumper or frame mounted tire carriers, but they would be nothing compared to the EVO, Genright, Poison Spider, or similar carriers that attach to both sides of the body and have their frame on the outside of the spare.
Just for comparison, here's a thread on the Maximus carrier:
http://wayalife.com/showthread.php?21478-Maximus-3-tire-carrier-installed! I was thinking about getting this one (I will be getting their winch mount and stinger, and liked it's modular design for accessories), but the Crawler Concepts just gave me another on to think about, and it's less expensive. Hmmm...