The HOKA Carbon X is essentially a Clifton with a carbon plate in its midsole and lightweight, performance-focused cushioning.
As a racing shoe, it's a harder ride than the Clifton and that's fine. Because, with the HOKA Mach, you're going to be going fast. Really fast.
That isn't to say that the Carbon X is uncomfortable; on the contrary, as a racing shoe it has one of the more forgiving impacts. As distances increase, HOKA's rocker geometry and carbon plate help to reduce fatigue. And, while it's slightly heavier than some other notable carbon plate running shoes, the Carbon X is durable and can stand up to punishment.
The combination of comfort, energy transfer efficiency, and durability at the cost of a little extra weight makes the Carbon X a little difficult to classify. It is certainly quick enough for short races and comfortable/durable enough for long runs, but you probably wouldn't want to set a personal best time for any marathons in it. Instead, the shoe will serve you well as a long-distance trainer.