Is this true: YES
Will it be a problem in the mountains: Those who use it (I have not yet purchased) say no it's not.. Car has to push fairly hard before brakes engage.
If it is.. You can adjust the cable length or add resistance.
NOTE: One problem with that system is the cable on the car. It is like the parking brake cable, Same basic construction. Every time you hook up and ESPECIALLY the first tow of the season Make sure it slides in and out easily IF it's binding bad things happen.