First off, 3 totally different sports IMHO:
There's the track and road, there's trail racing (could be cross country or ultra marathon distance)...and then there are the mountain races. All can be very painful and hard of course.
Let's put the vertical K and SkyRunning races (although usually shorter than an ultra) in the Hard Rock category with all the other Euro-style mountain races. These often involve steep slopes, a lot of times at altitude and a lot of power hiking. They also can involve steep descents that are off-trail on gnarly rocks (technical trail) or lose sand/snow etc. This is Killian's niche.
Then we have 3 main systems that are being challenged in the body (and this is a big generalization):
1. The Aerobic System (heart, lungs, Vo2max, Lactate Threshold, etc)
2. The skeletal muscular system (muscles, tendons, bones etc.)
3. The metabolic system (i.e. fat burning, nutritional stuff etc).
Now these three systems all work together and influence each other of course during training and racing.
So what makes Killian so dominant? Well, for system #1 we see he has a Vo2max of around 90 (off the charts). I don't know how it was measured but guys that ski often have a pretty high VO2max (he is about 5'7" and 125-130lbs if I had to guess).
Now system #2 is where you see a big difference (IMHO). A lot of these mountain runner types (I do not consider myself a mountain runner type btw). Have developed (or were somewhat born with) the ability to move efficiently through rocky steps, on 20% slopes, up and down mountains. Through lots of vertical change in their training their leg muscles seemingly do not break down from flying up and down mountains for thousands of feet on end as the hours tick by. They also have likely developed neuromuscular pathways for running economy (or shall I say power hiking economy too) for conquering steep hills and uneven terrain.
The same strength that allows them to move really well on this type of uneven terrain up steep slopes is a curse when it comes to the running mechanics required to run on a flat and fast surface (i.e. road, track or any smooth trail for that matter). I think we saw what happened at UROC, and again we probably will never see Killian at The North Face 50, or Lake Sonoma, or Mt. WA even (or any road marathon etc) because it's too "runnable."
But on the other hand if you're a speedy track guy please come to Boulder and try to "run" up Mt. Sanitas from the very bottom to the summit in 14min (Killian's time) and see how you fair....(hint: it's a fairly technical trail).
System #3 comes down to the ability to burn fat as a fuel for ultra events, and to be able to stomach food while moving/not get dehydrated a really high altitude (in the case of Hard Rock). A lot of shorter distance, speedy track and road runners are more carb burning machines but this system can be trained for (i.e. Max King at Western States this year finally going the 100mi. distance and not crashing and burning too bad)
As I said in earlier posts on MUT Running, course profile is pretty much everything in these mountain-ultra-trail races. You don't get that on the track or in road marathons where the numbers are standard and you know what to expect. Totally different sports.
That being said, I've always lost to Killian head to head (Transvulcania, Sierre-Zinal, UROC). But then again, I was only mid-pack at the Olympic Trials..
So how about this for a thought provoking discussion though: who's getting paid more..Killian or Coburn? (I don't know) Consider Killian has 375,000+ Facebook fans and Emma has just under 9,000 right now....