My understanding is that China used different hardware for their system than the JCI/Visteon version used everywhere else. That was why it had CarPlay. Now if they ran unified code across them all for the same interface, that might explain why it would brick the system.
Your understanding is wrong. The hardware might be marginally better (newer revision of chips or higher frequency or larger RAM chips or different wireless/bluetooth), but it still uses the same software as other chinese mazda cars equiped with CMU maybe with different partitioning of the eMMC flash chip. What is different is the interface to the display. That being said, there's a 90% chance that the Carplay application running on the CMU that runs on the chinese mazdas will also run on Mazdas with CMU from other regions eventually with a small hack (such as an AS-BUILT check that the binary does before-hand).
Don't confuse board revisions, with bootloader, flash variables, kernel or userland. The userland is 100% compatible between them. The kernel is probably identical, the bootloader is also probably identical. The flash variables might be different for different display output configurations.
I've also heard on these forums that CarPlay won't work because the current CMUs are lacking the authentication chip needed for communication. That is again untrue, because it's the same chip used for iPod Accessory Protocol that is already implemented in CMU. If you can play music from your iPhone via Lightning cable, you can also do CarPlay. If you want to implement your own, sign an NDA, go to mfi.apple.com and register and you get not only the technical specs for the protocol but also a sample implementation.
If you want to also do Wireless CarPlay, implement the Wireless Accessory Protocol (you have a complete implementation available at Apple that works with Linux) and you're all set, except that you're lacking Bluetooth Low Energy for service discovery.
Maybe you also want to do AirPlay for music instead of Bluetooth A2DP in case you make your car a wireless hotspot (see below). Sure, you have another excelent implementation available from Apple on mfi.apple.com.
Wireless hotspot: In firmware 33.something the networkmanager equivalent for CMU also supported 3G connections (from a very limited number of modems - a Nokia and a Huawei) and also had a hidden setting in the JCI XML files to enable it and to create a WiFi AP that shares that internet connection. Why they did nothing with that is beyond me. If they also added a remote start feature to the CMU, it would have been brilliant. It's sort of pointless now, because there are some 4G USB thumb sticks that act as routers for both the USB host (emulating an USB Ethernet connection) and via WiFi for the other devices.
My take: Mazda messed up the WiFi/Bluetooth shielding on most initial CMU cars and that hurt them big time. It's the scratchy sound that's happening during a bluetooth call. Replacing all of them is expensive for them. Since the customers don't have Wireless CarPlay or AA they don't feel the lack of a usable Wireless and they are hiding this under the rug, at least until warranty expires for the initial batches of cars (I have a 3 year manufacturer warranty over here).