When combined with a properly designed reflector, they actually work quite well. The problem here is, this was for a headlight design by Mecedes from a little while back. Now manufacturers of these kits think that they can utilise it to make sales and don't realise that the glare shield has to be placed appropriately at the very least for it work. Then comes the issue of the reflector housing that it goes in, which they can't cater for everyone.
Edit: For example, these lamps here are H7 and you can see where the shield in in relation to the locating tab:
Not all cars that have H7 bulbs have the locating tab at the bottom. Some have it on the side (either 45 or 90 degrees to the vertical)... or even worse, the top. If the locating tab was at the top, so would the glare shield. This would result in lower light output and glare problems still there.
A good example of how most of these are just for marketing, they state on their product listing:
When in fact this statement is incorrect as the glare is produced by light emitted from the bottom side of the lamp reflecting on the bottom half of the reflector upwards towards anything in front.