A real pain
I got stuck using a mates one and was about ready to throw it off the cliff by the end of the day. Climbing gear needs to be simple and fast to use. The few extra steps needed to rig it waste valuable time and increase your chance of dropping it. The same can be said about racking it on your harness.
I also don't see much of a benefit. You are much more likely to be injured by an incompetent belayer than to be injured because the biner was cross loaded. It is also easy to rig it incorrectly so that both the device and the belay loop are in the big end of the biner.
If you really are worried about it, such as rope soloing, then buy a steel mallion and save your money