A large number of the updates sold by third parties are counterfeit pirate copies as almost all manufacturers have nailed the distribution down very tightly. I am not saying they don't work, but they are certainly on the dodgy end of the scale. If you are going to go down the dodgy route, then you might as well just pay £20 on
eBay as at least you have some comeback and if you want to you can copy the data and then report the product you bought as counterfeit and get your money back

.
The official updates help pay my salary, so I would encourage you to buy them, but I fully understand why people don't when the mark up the automotive companies add is ridiculous (about 10 times what they pay).