One of the challenges of harvesting worm castings is getting a consistent product in every bag. The biggest variables are the quality of the bedding and the purity of the worm's food source.
Many lower-quality worm casting harvesters use a "flavor of the week" food source for their worms. A lot of them simply have a big compost heap with a bunch of worms in it, and then call the finished product worm castings. Tisk tisk tisk...
The problem with their method is that you don't really know what you're getting. One batch might be full of pesticides, while the next is full of herbicides and other chemicals (watch out organic farmers and gardeners!) One batch might be high in one particular nutrient, while the next is low in that same nutrient.
This results in an inconsistent product where you never know what you're going to get. It might work great for your plants once, while it doesn't do anything the next time.
If you're planting a garden or growing crops, can you really afford to gamble on the quality of your soil supplement?