Disadvantages

Loss of proximity to the participant limits what the test moderator can see directly of the proceedings. It is possible to miss subtle behaviors.
It is crucial to have a good angle via the video camera on what the participant is seeing and doing, because you may be blocked from seeing directly.

Because the test moderator is not within the participant's peripheral vision, if the test moderator is directly behind the participant and within 10 feet or so, the test moderator could make the participant feel very uneasy and overly self-conscious. The guideline here is to remember when you are behind the participant but within ten feet, you must stay within the participant's peripheral vision. Farther back than approximately ten feet should not cause any discomfort to the participant. 

As with the simple single-room setup, there is limited space for observers.