Could be, if you find one that hasn't been abused to badly. Rental cars are typically abused, especially mustangs. Convertibles aren't abused as much, they get mostly used as cruisers.
Spin the wheel and take your chances on a used rental car. Multiple drivers, lot attendants, daily gas refills at multiple cheap stations, fast harsh multiple car washes, scuffs and dings at tourist destinations, and lots of short driving cycles. And maintenance......who knows? The convertibles come from warm weather locations before going to auction. My brother in law rented a triple black GT convertible in Hawaii so they do exist and he drove it hard.
My 2007 Grand Marquis “program car” actually was a rental. I found a paint and body repair ticket for the rear bumper cover under the trunk mat that gave it away. (That was not the last time someone has decided it was too long in the parking lot.)
I lucked out as it has been very reliable, and still drives great and looks almost new. The repairs, other than PM, have been Honda like rare, and less expensive. I’d take a chance on another, newer Ford’s reliability if it did not show signs of being wrecked or trashed.
I wish. I got my BMW 335 from Hertz Rent2Buy like a decade ago and couldn't be happier. I wasn't too concerned with people driving it hard, because I'd only push it that much harder, considering that a good workout. After changing things like the brakes, and the fluids to better quality aftermarket ones, I'd call it a day and keep up with the thrashings, apparently it's all the car would know.