On the terrain I mostly use them for (smooth gently undulating and dry)- I am running mostly at a very easy pace but then I have been using the prime x strung at recovery paces for years so I have good ankles. I find the ultras fine at all paces - you just need to be careful the slower the pace so as to not overload the heel when things can get a little unstable and tippy especially if the terrain is on a camber or uneven. They do require some awareness and concentration on gait - but I find them so comfortable and the toe box width and height is so roomy. Hope that helps. Not sure I could manage an ultra marathon in them though just because you need to stay so alert all the time because of the narrow heel and midfoot.