Nothing is
particularly special about the Broadwell X1, in that it isn't better than the subsequent models (thajt . It was, however, point where the X1 line hit its sweet spot and stabilized, while also having the first Intel iGPUs to have the oomph to drive multiple HiDPI displays for non-gaming workloads (technically Haswell did as well, but the Haswell X1 had an "experimental" keyboard design that utterly sucked, including a touch bar four years before Apple!).
If you look at the X1 line since then, it hasn't changed much except incremental CPU upgrades (and that's a good thing, the design works!) and fixing an occasional generation that had lackluster speakers or a lackluster maximum screen brightness.
In an era before Dell's "XPS Developer Edition" line hit its stride, one reason you would get a Thinkpad was that so many developers of Linux had one. As a Linux user (versus someone actively contributing to DEs, WMs, drivers, etc.) you benefited from their hard work put into fixing their personal annoyances with drivers. Remember this is coming out an era where the very idea of a Laptop's WiFi working in Linux was a dice roll. Issues still exist, for sure, but things really have improved since the 2000-2010 era.
In every case, there is always a chance of warts. Picking a specific distro or DE or version thereof may still bring more pain than others. Nonetheless, my personal and direct experiences with Dell XPS (13-inch, no dGPU), Thinkpad X-series and T-series, and System 76, have all been positive since about 2015. Total number of systems configured and supported (loosely, for friends over time) with each is over 5 in each category. At work we have many System 76 Galago Pros in use. They do good work, and as others have noted you can get nice things like 32GB of RAM. The physical fit-and-finish is still a half-step lower than Dell and Lenovo (imo), but the overall value is good, and we still use them partly to support such an awesome, Linux-focused company.
Again, there will be occasional warts [0], but far less than getting an MSI "gaming laptop" that has better hardware specs for cheaper, and trying to run Linux on it. It seems like a good deal, but then and after fighting Nvidia drivers for hours, you finally get into a working environment, only to find that keyboard function keys may never truly work (just one example among many, from many tier 2 brands, that I have personally helped troubleshoot), or some other deal-breaker in the experience.
I have no direct experience with Purism, unfortunately.
[0] An example of a random wart: even though I have helped set up four Thinkpads with Ubuntu-derived Distros (Mint, Elementary, Ubuntu itself, PopOS) in the last 2 years, a friend recently had issues with stock Debian on the latest X1 Carbon (I was not involved, but he is a Linux guy who knows what he is doing). Mint just works on that model, but he prefers the free-as-in-freedom of Debian and ran into some snags.