Worse yet, it will be a joint US/Mexico Customs facility and the Mexican customs office will not be subject to US laws. Obviously there is a U.S. Sovereignty problem and that plan has not been fully approved, only proposed.
Incoming cargo, mostly from Asia, will hit North America in Mexico and be subject to Mexican Customs. Currently, the docks and facilities where the incoming cargo hits Mexico are being funded and set up by a Hong-Kong shipper.
According to press reports, there will be an inspection done at the US border by Homeland Security. Historically, they spot check and rely on Mexican Customs.
US exports to Mexico will be inspected by Mexican customs in KC. US imports from Mexico will be inspected in Mexico by US customs.