I used to build a detector for explosives in academia that is now being marketed to the TSA. To get funding for these projects, its pretty standard to rip another technology apart, point out all of its flaws, then argue why yours is better. But every technology has their drawbacks and ways of skirting around it. Today, most technology deployed is more as a deterrent and less as a counter-measure.

You want to know another way to hide illicit items in those body scanners? Surround the items in water.

There are also certain materials that give off false positives and most likely the software will scan the image response against a bank of known materials. I wrote pattern detection software that did just this. Oddly enough, cheese is one of them. So stick your knife in a nice big piece of cheddar and you might make it through just fine.