That can happen. I have flown through MIA twice within the last month or so, once arriving from HAV, once from SXM. Lines in immigration (and customs) were quite lengthy for the first (AA 9413, arrived 11:08 on a Saturday), not long for the second (AA 748, Arv 11:35 on a Monday); whereas Global Entry was easy.

Proceed past immigration entry to GE kiosks, process, take receipt, proceed to GE queue (do not confuse it - it's on the left - with the Special Services queue for those in wheelchairs).

The most significant problem with GE was several non-GE, non-US citizens proceeded past where they were supposed to enter immigration and attempted to use the Global Entry queue, lengthening the time for the queue to process while they were repeatedly told why they could not use the GE line.

Without GE I would not risk getting caught in the lengthy queues and chaos that can easily occur using MIA as a Port Of Entry, particularly for non-US residents and citizens.