I had a X1C bought early 2018 and it had great battery life. And I generally was happy with it. Unfortunately it got stolen (luckily I had full disk encryption). Without much investigation I took a X1C again. This time it was a 7th gen. It had HiDPi display which was a bit of pain to have all apps working with a satisfactory resolution and the battery life is much poorer than before. I would take my older one back right away if I could.

Edit: The author uses an 11 year old machine. Not a surprise it works well. With all the new stuff the vendors introduce difficulties are much more common. I hear a lot of complaints from colleagues with Thunderbolt docks, the newest Intel camera generation has no Linux support, not that much has changed. Whether it's 2 steps forward and 1 step back or the other way round is debatable.

With TLP installed and properly configured? Strange, battery life of my x1c tends to be as good or better as on Windows.

The battery life of hidpi monitors is often much worse than the same machine with a lower resolution monitor.

Yes, but it doesn't have anything to do with operating system installed on such laptop. Also, not all of x1c's are equipped with such displays.