I have a 2017 Volt and an iPhone 7 Plus. The iPhone is both paired via Bluetooth and typically connected via USB to CarPlay. This way, if the CarPlay connection fails for whatever reason, it fails over to the Bluetooth connection. I have found that if I let the car connect to the Phone via BT first, and then plug in the cord for the USB/CarPlay connection, things are a tad more stable than if I plug in first and attempt to have USB/CarPlay connect before BT.
Of course, YMMV.
BTW, does anyone else have a problem whereby the car switches to the iPod/iPhone input on its own and refuses to stay in any another audio source (like XM)? I usually listen to the XM source but still have my phone plugged into USB for CarPlay. Most of the time, the car stays on the XM audio input and everything is great. Randomly, though, the car will automatically switch to the iPod/Phone input and no amount of switching back to XM "sticks." This happens completely randomly and I can't find the repeatable set of circumstances to have it happen or prevent it from happening. All I know is that sometimes, I have to resign myself to listening to audio from my Phone instead of XM because I can't get the car to stay on the XM input without completely disconnecting my phone (unpair, unplug, or power off).