I run linux on a P1 (2nd gen) and I know people who run it on the X1 Carbon.
The linux experience with the P1 has improved quite a bit since I got it about 8 months ago as support for the hardware has improved in the kernel.
Generally, I would say ThinkPads support linux as well as anything else right now: the ultrabooks with Intel graphics are fine. Anything with optimus graphics chip will work but not quite perfectly. Imho stay away from OLED for now.
Ubuntu 20.04 will bring fractional scaling and a new kernel. Along with packaging the nvidia drivers on the ISO and the optimus support they've built in, that is probably going to be the best laptop distro.
Been running Fedora on an X260 for a couple years. ThinkPads generally have good support with the cutting-edge distros because of their popularity (which is because of their robustness and repairability).
I have a T480 with Ubuntu 19.04 on and it works fine. I chose not to have a fingerprint reader or card reader, so I can't tell you if they work, but as a work laptop it's build very well and I can see another 3 years of life at least out of it. Business support is also nice, with on sight if needed (which I think you can also get with Dell).
I have the X1 Carbon from 2019 and it mostly works fine in Linux but the audio has been a major issue. Getting the microphone to work at all requires new drivers and a lot of tweaking that took months after release to become usable.
The new drivers are still quite unstable and many people still have no mic or bad audio.
