> What would prevent people from disassembling this, replacing a few parts and making new gun? Making hand gun is very simple once you have basic components, such as barrel and bullets.

Nothing, but why would you want to? The whole point is so you can buy a gun that only you can use (and not e.g. your kid). I guess a thief might still steal a gun and then disable the system, but making life harder for such a thief is a good thing even if it doesn't make it impossible (just like an engine immobilizer). But if you're worried about "bad guys", the point is just to stop accidental shootings. There's nothing in the system that would stop you deliberately e.g. shooting a cop with one (as long as you had the wristband on).