That's one of the fun things about standards used in products made by an array of companies. I can't tell you how maddening it has been to set up some of the Bluetooth devices out there (and remembering which "side" I needed to start the pairing from).
In the end, you need a good standard and then you pick the hardware based on brand, cost, design, etc. I particularly like the Wayne-Dalton Wireless Gateway, and would use it as my Primary remote control. To add a device into a scene, you just hold the scene button (1, 2, or 3) and press the button on the device. If it wasn't in the network, it's added in automatically. Simple.
Then if you want to have your car activate that same scene, simply (for instance, with your new GM vehicle) press the button the Wayne-Dalton Wireless Gateway until the red LED goes off, and then hold down the respective garage door button in your car. The light on the Gateway will flash, telling you that it's good to go. Awesome.
BTW, there are three or four very exciting announcements coming at EHX. Please feel free to contact me offline if you'd like more info...