I use Waze several times a week in my 2018, and have never had an issue with it.
There was a lot of GPS problems when iOS 12 launched. It wasn't really hardware-specific, the problem was with the software. I have a Tmo X, my friend sitting next to me with a Verizon X late summer and we competed whose phone would display the weirdest locations. Also, this problem appeared on other iPhones, not just the X.There are two versions of the iphone x. If you have the unlocked/verizon one then you have the better GPS, if you have the intel one that is sold with ATT and some others, they've had a lot of issues with the GPS in those. If the GPS works except for in the car then I suppose it could be the car, but most likely its the phone.