It is not effective because WB change from corner of the room to corner of the room, seconds to seconds, milliseconds too, depending on mixture of lights, nearer window you have daylight, some room mix fluorescent that changes every milliseconds, some mix in different bulbs of tungsten, or different age, many places are have started using LEDs, which makes things even more interesting. Some events have rotating lights of color. And when you add in your own flash and your varying shutter speed will influence colors again.
There is no magic lenscap that set best white balance. In fact it only mess it up for you. If you are shooting fast changing scenes like an event, modern day camera gets WB way better than that lenscap, immediate on the go, adjusting to every different scene. If you are shooting a controlled scene, better to set in post production as you will be changing anyway. If you are shooting forensic and needs get true colors, a color checkerboard is better.
Dont waste money.