Been on Carnival lately? It is in many ways superior to RCI although on average they are equal.
However, enough thread drift. The topic is debarkation in Miami and not a debate of the merits of one line over another. Customs decides on the processes used for debarkation and not the cruise line. When a cruise ship - of any line - arrives at the debarkation port it is under quarantine by Customs and Border Protection. The clearance process can depend on the ship and the terminal logistics for CBP.