Nothing comes for free.
For example, one way that a network can guarantee never to lose a packet is to limit the users to a rate that is so slow that even if they all conspire to send data at the same instant, they can never overload the capacity of your network. Of course all the users in the world never do conspire to send data at the same instant, so your network is never operating close to its possible capacity, but that's the price of guarantees. ISDN works this way. After you've paid all this money, ISDN still doesn't really guarantee anything. If you leave your ISDN line up for long enough, you'll find that eventually something always goes wrong and the connection spontaneously fails.