More thoughts to add to this conversation...
1.) Xs and Xs MAX support B14 on AT&T, which none of the prior iPhones did. Is everyone with AT&T that's having issues confirming that they are not connecting to different frequencies?
2.) Same as above for T-Mobile, but with B71...none of the previous iPhones supported it. Once again, are we sure these phones aren't connecting to services that might have issues in the areas where other bands don't?
3.) Verizon has released multiple carrier profile updates since iOS 12 was released, has anyone done a side by side with an X/8/8+ with the latest updates to make sure those phones aren't experiencing the same thing now with possibly different band steering?
4.) There are plenty of people out there (myself included) with zero issues, and have directly compared to the older models. There are also a handful of people that claim to have had a bad device, and swapped for one that now works properly. That said, there have been zero changes to the devices themselves (otherwise someone like iFixIt would have found it by now), so what theories are there on why one device works and the other doesn't?
5.) Am I alone in believing that the masses look for issues when they see that others are experiencing issues? I have been guilty of this myself many times before, but I truly suspect not many would be pointing the finger at a phone issue if it weren't for a 110 page thread riddled with complaints of lower signal that was started by an amateur blogger who decided to attempt to decipher FCC test reports. Antenna gain is only a portion of the equation when it comes to RF performance, and anyone that understands any form of RF transmission should know that.
6.) iOS 12.1 should be released on Monday or Tuesday. I would LOVE to see Cellular Insights run another test comparing 12.0.1 vs 12.1 on the Xs, as well as obtain a Qualcomm X to compare with the original test where an Intel X was used. I know they said software updates had mitigated any differences between the devices during the iOS 11 run, but I would still like to see them put the reception issues to bed with actual controlled testing.