"I've heard it said that US trails are generally not as technical as the ones in Europe. Therefore US athletes are not as good when it comes to dealing with the tricky bits of the course.
Dunno whether this is actually the case or not…"
Sort of depends.. where versus where..
In the alps the UTMB is very untechnical.. very quick running just long gradual ascents and descents, you are going uphill for hours then down hill for a good hour plus at a time so it really hammers your muscles.
Technicality wise I have to say the trails in the NE USA are about as rough as I've ever experienced, even compared to the majority of UK trails. Out West USA the trails are said to be less technical.
Races like the UTMB, Sierre Zinal are very much mountainous trail races, with quick fairly non-tech sections and the odd rougher section, but the sky races generally cover much rougher, almost trackless terrain.
I think the issue for US runners is similar to the ones faced by top UK runners, the terrain is just foreign to them, we just can't replicate easily those sorts of profiles they face. Some have done OK but as Sage says they are the ones who get out there early or have a great deal of alpine experience.
The speeds in the UTMB, even for the elites, is slow, I don't think its down to speed like for example the more runnable 100 milers. Lizzy Hawker will beat many of the men who are much much quicker runners. I think its basically efficiency and wear and tear.. basic robustness..
re Krar, not sure, I got the impression from racing him he's not great on the rougher trails, they are rough enough that if you aren't super efficient at traversing the rougher trails you get more more muscle damage and in a 100 like the UTMB I think that's crucial. Its where Kilian murders the field, at races like Sierre Zinal he's just one of many top runners, at the big mountain ultra's he's pretty much in a class of his own, well maybe with 2-3 others.