It is never nice to underestimate what can be done, I would prefer to have all zwave devices with encrypted data transmission. It would cost probably not so much more but give a peace of mind.
If somebody invests this amount of energy to mess with your HA setup, you probably have set him up enough that he just takes an easier route to get you into trouble...
So I agree that it is necessary to think about attack vectors, but paranoia does not help any further..
As possy said, if you need higher security (for whatever reason) use e.g. KNX, but then better make sure that the bus cable is not accessible from the outside (e.g. in the garden between buildings, ...)