Most omnidirectional antennae have two factors that work well for them.
1) They have fairly symmetrical and uniform RF profiles. This means that you are less prone to sudden drop-outs in the video as you change the orientation of the antenna as it is less likely that you will line up well with an area of lower RF intensity.
2) They are often circularly polarised, as the mushroom is. This means that that the correct Rx / TX combination effectively cuts out interference from signals reflected from various obstructions, making for a more clean and reliable reception.