Problem is then you're depending on a rental, which your (alleged) reservation doesn't actually guarantee.

You always depend on the business to provide the service reserved whether it is plane, train or automobile. I'd worry more about the flight into/out of MIA than the car rental.

Rental car agencies oversell far more than is typical of most industries, including the airlines.

Airlines, hotels, car rental agencies all overbook as a matter of course. You assert that car rentals are worse than airlines. But you cite no numbers, no sources, so... Maybe?

Not sure it is even the best comparison since we are talking reliability. Airlines manage overbooking very well, but flights delays and cancellations are the reliability issue.

By the time you're done paying for insurance, the nickel and diming on the rental, the tolls, and the physical toll of driving long stretches in a country you are not familiar with, it starts to sound like a better option to take a stress-free train ride and then a Lyft to your destination.

Well, last time I booked a car last minute (from one of the big ones, I think it was Enterprise), they confirmed my confirmation but didn't have any car available when I showed up the next day. I'd imagine the train is somewhat more reliable.