I have seen the bungee through the zip tie done, and my thoughts are that if it is tugged on hard enough, it will likely pull the whole mouthpiece off, with the fisherman's knot it still takes quite a bit of force to pull it off, but perhaps if something is pulling that hard on it, it is better for it to come off.
However, I don't see zip tie method vs fisherman's knot method as a major issue or really any issue at all. If something is snagged on my backup, I would mostly likely remove the bungee from my neck and fix the issue and if someone is going for my backup, they would have my primary in their face before they were able to tug hard enough to remove it (or to break it with the zip-tie method). I can't think of anything else that might tug hard on a backup.