My main concern with all digital systems is the ease with which they can be jammed. For example you can get a mobile/cell phone jammer the size of a briefcase that will take out a whole street.
It is a similar situation with GPS - even if deliberate jamming is unlikely accidental jamming does occur.
"Man-made interference can also disrupt, or jam, GPS signals. In one well documented case, an entire harbour was unable to receive GPS signals due to unintentional jamming caused by a malfunctioning TV antenna preamplifier. Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight"
And read this about home made jammers.