I think one of the toughest things about their trip was likely just getting tired of being in the cold all the time. In a normal thru-hike you can have times, sometimes quite extended times of cold, but can look forward to walking into warmer conditions later.
Even on more "normal" thru-hikes, I've found myself becoming sort of mentally weary of having to tough out repeated snowy frozen mornings. For these guys, it must have been just day after day after day of getting up to frozen mornings with no hope of respite until the end.
Another factor that's easy to overlook is the amount of daylight available to you in winter hiking. Harder to do the miles when, not only are trail conditions tougher due to walking in snow in general plus likely additional navigation issues (akin to some parts of a CDT thru-hike), but when you have a lot fewer hours of daylight to do them in. And of course that leaves off the really uniquely tough and dangerous aspects of winter backpacking.
My hat’s off to them. But not for too long, as my head gets cold when I take the hat off.
