A mixed system is safer than a non-conductive system
Since the rest of the filling station, like delivery and dispensing hoses and the fill necks and dispenser nozzles are conductive, it is the introduction of non-conductive piping in between that makes the system "mixed". All the conductive and dissipative parts of the fueling system can be grounded to earth and any static charge will dissipate so quickly that it never becomes a hazard.
In non-conductive pipes more static is generated during the normal operations of the filling station, than is the case with conductive piping. This static cannot dissipate at the rate at which it is generated, so charges accumulate and can become an ignition hazard. This is the nature of non-conductive objects.
The less non-conductive piping there is on a station, the better, since less static charging will occur and it cannot accumulate. If conductive or dissipated piping is installed at the end of a pipe run, this section of piping is grounded to earth and one source of static ignition hazard is eliminated. This type of "mixed" installation is safer than a pipe run that is completely non-conductive!
For those who want to avoid mixed systems there is only one way: Use conductive piping all the way.