My two cents: Even as a fluent mandarin speaker (have lived in Taiwan now for 10 years, been to the mainland at least 30-40 times), I try very hard to stay away from black cars that approach me. That said, I always ask whoever I am with (customer or supplier) if they have a car/driver that they could call for me. As mentioned above, often the cars themselves are much better condition (Passats, Toyoto minivans?) and drivers like this will never want to screw you. They depend on their honesty.
Taxis in general are often uncomfortable, smaller (jetta or santana or worse). The one redeeming quality is they are registered and the chances of something going south are extremely low.
I mostly let the doorman call their cab --- even if there is a kickback, so what? I would rather the hotel be on my side for supporting their "friends" in case of an issue. Some doormen have told me (in mandarin) that "that other cab company always seems to try to screw people so we strongly suggest you wait for a guy from our recommended company". Might be BS, but you can only do so much to reduce your risk.