With the majority of issues I read about them, it's not the fact that they screw up from time to time, because as you said, most companies do, especially with a high volume of orders.
It's more the fact that their customer and after-sales service is really awful - they are slow to respond to customer service requests, make you wait nearly a month for missing orders, rarely get a pre-order to you within days 1-3 of release, frequently cancel orders due to "pricing mistakes" (how many times does a pricing mistake have to occur before you decide to change your process to stop this from happening) and there are a lot of reports of opened and scratched games being sold as new.
I agree that all these issues are not mutually exclusive to Zavvi, but they are really bad at responding to and resolving these issues in a timely and satisfactory manner. Quite simply, in terms of communication, service and satisfaction, they are a world apart from companies such as Amazon and shop.to, and that's why I'm always happier to pay a "premium" of a couple of pounds to use what I consider to be infinitely more reputable retailers than any related to the Hut Group.