It's unstable to the point where it's basically useless; worse so for iPhone 13 users (including Pro/Pro Max).
Just recently with the 15.2 update am I able to use Carplay with ONE of the USB inputs in my Jeep Gladiator (one on the dash/USB1) and not have it crash. The second I switch to any other USB input in my Jeep, it goes back to crashing (I discovered the dash input works by accident honestly). I know it's not a problem with USB ports b/c my GF is on iOS 14 and CarPlay on her iPhone works just fine with any of the USB inputs in my Jeep. Same for my other friends on iOS 14. If I switch from the input on the dash to the one in the Center Console (USB2), which the input I used primarily (less cables laying around, cleaner and neater appearance), it crashes every 3 mins, does not matter the app. It even crashes this frequently if I'm not using an apps at all. Same for the rear USB inputs.
Whatever version of iOS they shipped iPhone 13 Pro Max's with (not iOS 15, as I had to upgrade to that after about a week of owning), was perfectly reliable. After I upgraded, it started crashing repeatedly in every car I drove that had CarPlay (all with factory CarPlay systems that were verified by Apple to be officially supported). TBH, the features they rolled out on iOS 15 were not worth sacrificing CarPlay reliability. I have done all the online "troubleshooting" and all the troubleshooting with Apple Support. Nothing. I've even done a full wipe/restore of my iPhone 13 Pro Max AND had the phone replaced, same thing. It's for sure a software issue and Apple has admitted they have no idea what the problem is.
Frustrating as hell because I use CarPlay almost daily in one or both of our vehicles. Particularly terrible if you are relying on it for navigation in an unfamiliar city or town and you miss turns/exits because CarPlay froze up and you have to disconnect and reconnect your iPhone to get it working again.
If I could downgrade to iOS 14, I would.