I've rented a fair number of cars while traveling in the last year and hit a number of problems:
* Everything Ford makes has a bug where the car's computer will lock for a minute or two and either crash / decide the phone doesn't exist or display a blocking modal dialog telling you that you have too many tracks on your phone if you connect it using USB. I have ~11K tracks (iTunes Match) and that's apparently more than they tested with. Toyota had a similar failure with USB.
* Bluetooth is more miss than hit: pairing is usually buried in the UI, slow, and reconnects are unreliable so you might have to try a couple of times before anything connects.
* None of the cars have a UI designed for more than a handful of tracks so while you can get something to play eventually, the experience is terrible.
* Not everyone supplies enough power to charge over USB or does so inconsistently, so you may arrive with 10% battery if you used the GPS.
100% of vehicles with AUX in lines work perfectly every time without any fussing. I want something like Bluetooth to work but there's this massive market failure where almost nobody seems to have a QA team or a support process where updates actually happen (BMW suggested that a friend buy a newer car to fix their bugs). I'll give Subaru an honorable mention for having reliable software but they desperately need to hire some UX specialists.
