I have not used it myself, but I have two regular riders who uses it all the time specifically for WDW. They go to WDW 4-5 times a year, and I drive them to and from the airport.
I don't know about "guarantees," but they are regularly scheduled buses. My friends have not been late coming back.
Don't cut things too close because Miami is a big airport and everything takes time. What airline are you flying?