Will Mobile Passport plus work 100% of the time?

No.

Travelers may still receive an error message while using Mobile Passport plus. The overall intention of Mobile Passport is to ease your arrival into the U.S. thereby improving associated administrative and operating functions. However, due to various security reasons, U.S. Customs and Border Protection (U.S. CBP) may use their discretion whether to apply certain procedures as it deems necessary to determine access through the inspection area. Mobile Passport members may still receive an error message indicating that their submission has not been successfully processed by U.S. CBP.