I should add that in my opinion, many trails with a reputation for being challenging are not difficult because of the terrain. Toss in cold temperatures and precipitation, though, and that’s when you might have more of a challenge than you bargained for.
Hiking the AT through the Smokies comes to mind as an example of this. By itself, that section of trail has some significant ups and downs, but any fit hiker should be able to handle them without being over-challenged. When the weather becomes cold and rainy or snowy, as it often is, that ordinary trail becomes troublesome and perhaps dangerous for even the most experienced hiker.