In my experience, BT is always hit or miss with devices and phones. Yes, I have seen issues with previous Nexus devices, and I have seen problems with Samsung Galaxy, HTC, Apple, etc. Every new phone and every new BT device (or vehicle) is a wonderful new opportunity for a hardware conflict. Google typically adheres to specs perfectly, it is the rest of everyone that doesn't, that cause issues. Google will need to step in here and do what they can to smooth this out, just like Apple does, so their BT stack works with as many devices as possible, and be damned about the specification adherence. My $.02.
That said, unlike previous Nexus devices, Google IS actually paying for licensed drivers (like the LTE voice codec thing). I expect things to improve here.
Lastly, my Pixel seems to work fine with my Toyota Camry 2015 (non nav setup). I do get occasional hiccups on the audio stream, maybe 3-4 times in my 45 minute commute, that I cannot explain for. New OS, new phone, not all apps upgraded... like all new OS releases, it might take some time for the details to smooth out.