It seems you have been lucky. I utterly dread MIA CBP. Three hours is something I can easily imagine happening - I've had 1hr30 - 2hr waits, with that looping tape of Welcome to the USA playing over and over in the queue. Then ah relief, a desk, oh wait, another 15 mins at the desk with the most obstructive officers I have ever met. It's unusual, as at every other PoE it's a "hello, purpose of visit, 4 fingers please, stamp, have a nice stay". Oh, and then last time (from PTY - probably explains why), a full bag search at Customs. In my opinion, it's the most awful PoE in the USA.

If due to some mental lapse I actually wanted to go to MIA, I would easily consider connecting through another airport just to avoid MIA CBP.