I don't recommend a bungee net for a large backpack, unless you find one with really thick cords that you can pull tight. I've used a lightweight one which worked fine for a bag with small light loads. Anything bulkier or heavier risked flopping over to one side.
The real advantage to bungee cargo nets is in having a lot of straps, which lets you loosely strap something down and it won't easily slide out. It's convenient, secure within its limits. A big bag, or laptop bag for example, that needs to be strapped tightly by strong cords defeats the purpose.