The 13" MBP that is most comparable to the X1 Carbon also uses U series, while the 15" MBP is best compared to the X1 Extreme, which, you guessed it, offers the better H series CPUs.

Same for Dell XPS 13 vs 15.

You can’t have your pie and eat it, in exchange for battery life having U is completely understandable. Depend on your workload (mostly just a ssh frontend), even Y is acceptable.

I'm a big fan of the U series cpus: They consume very little energy, which leads to less overheating, less fan noise, and longer battery life.

(Conversely, the H series cpus consume a lot of energy: the laptop overheats quickly and has to slow down anyway.)

Now it makes sense why may 2018 MBP always gets hot while my previous 2015 only rarely does.

Thanks to constant throttling. Which makes an i7 feel like barely an i3. (i7-7660U in an XPS 13, at least)

I cannot understand this statement - if I need a (relatively) thin laptop my primary focus won't definitely be the number-crunching feature (more something like weight, connectivity, panel colors, input devices, etc...).