Maybe specific to ORD (at their new consolidated car rental facility), but on a recent visit the UC selection was miserable and sparse consisting only of a few Malibu, and a Ram.
I had an SUV booked which was the cheapest (less than intermediate), but chose the Ram instead.
On exiting the lot, the Hertz agent asked: "Was there an issue with your car, or was your car not in the assigned lot?"

I referred to UC, and that worked, but it seemed as if it was unwelcome.
Strange experience.