I've been using the cartridges for over a year, they have never leaked. If they did, I'm sure novo nordisk would be the target of many many lawsuits.
Which bit leaked with the ones you used? Maybe the pen rather than the cartridges was defective, and putting pressure on the cartridges in the wrong way or something. Or maybe the boxes had been stepped on or something, so the glass was cracked? Thought that would be quite difficult to do, I've dropped them and stood on them by accident before, without causing any damage to the cartridge.
I tried to take an empty cartridge apart once when bored, it took a lot of effort - I had to use my teeth to grip it to get the top off!! Even then it couldn't leak, as there is a metal bit under the plastic, the plastic is just for the needle to attach to not to hold the insulin in. I couldn't get the rubber seal at the other end out either, it seemed quite secure.