They might drop in value a lot when driverless technology goes mainstream. If the supply of sedans is the same but the demand is way less because driverless cars don't need to be wasted sitting parked 90%+ of their existence, that could make used cars way cheaper for a while.

Uber drivers are likely to accrue higher than normal mileage on those vehicles, which would lead to faster depreciation.

Loan payoff is usually gradual, so I'd imagine most of them from day 1 are underwater as far as what they owe and what the car would sell for on an open market.

Market similarly discounts used taxicabs, used police vehicles and anything that's re-sold by car rental places - such vehicles have higher mileage and are more likely to have been abused.