I read that also. I think the Cap can be pointed at the subject......need to do some more research.....
It's not a problem with the product so much as it is an inherent limitation to the whole approach. It is sort of like the problem with reflective light meters being fooled by black or white subjects and over or underexposing them.
As an example, let's say that you are taking a picture in a room. Behind you is the sole light source - a reddish/orange tungsten bulb. In front of you is your subject wearing a blue dress, with blue hair, in front of a blue wall, blue floor, and blue ceiling. The expodisc acts as a diffuser mixing up all of the light hitting it. The light is originally reddish/orange, and that's what you really want to correct for "accurate" white balance. However, the blue light reflected from everything in the room creates a rather cool bluish cast to the light coming through the expodisc. As a consequence, your white balance is off. You can fix the problem by zooming in on the light itself and letting the expodisc see only that. If you had a grey card lit primarily by direct light from the bulb, this wouldn't be as much of an issue.
I don't see how any form of on-the-lens diffuser will work any differently in this situation. At it's best, it can neutrally diffuse the light hitting it. At it's worst, it can introduce it's own color cast. The expodisc is a very effective diffuser (better at it than a typical translucent piece of plastic) and is neutrally balanced (or warmly balanced if you by the warming one). Aside from that, I can't think of anything short of magic that it really can do.
To accurately white balance a photo, you need to know the color of the light source for the picture and compensate for that. With an on-the-lens diffuser, the only accurate way to do that is to capture the light directly from the light source and not reflected from an object.
All of this also assumes that you have a single light source. In the real world, you might be blending white direct sunlight with blue skylight with green light reflected from grass. In that case, there is virtually no way to totally eliminate color casts. Even in the example above, if you compensate for the color of the light source, you will then be left with extra blue from the light reflected by the floor and ceiling. White balance in all but the most controlled circumstances is a compromise. The expodisc makes it relatively easy to get a reasonable and consistent compromise. So does a grey card, a white sock, etc. For me, for photography, the expodisc doesn't add enough value to justify the cost. For video, it's a better value. Reasonable people have come to very different conclusions.