Many systems can be jammed(bypassed)
If someone wants in bad enought they'll find away!
Yes, that is very true.
That is why one should never, ever, 'help' a potential thief by posting a sign up front of their house telling the thief the brand and type of security system they are using.
For example, if you post a sign saying "My house is protected by an X10 security system", the thief can easily refer to the "How to defeat an X10 security system" chapter in their "House Robbery for Dummies" handbook.
In other words, keep the thief guessing. If they feel unsure, possibly they will just pass your house up.