Do not carry a full carboy only by the neck. All of that weight is concentrated in the small area of that carrier, exerting a lot more force than the neck was designed for. It may not happen often, but the risk isn't worth the reward, in my opinion, especially when there are much better options to moving full carboys.
Some folks like to use milk crates.
I always use a Brew Hauler when moving carboys. It's safe and much easier to lift and move around than the handle. I even use them on my plastic Better Bottles, just because they're easier to handle that way, especially when you have to lower them into a chest freezer.