The main issue for me is control. You can only use voice commands if you have an internet connection, and holding your phone in your hand to adjust volume and skip tracks is today's premiere form of distracted driving.
So far, the best solution I've found for an unconnected device is to plug into a
miniDSP unit and control it with either voice (requires an internet connection), or a hidden bluetooth controller such as
this one. It seems to allow for a remote volume knob as well.action it provides.
There might be some shenanigans involving a USB hub to provide power to the device, but all of that can be hidden under the dash or in the frunk.
You do lose some quality sending audio over Bluetooth, but it probably isn't noticeable if the engine is running.