Imo they are not uncomfortable, but not functionally comfortable if that makes sense. I’ve currently got speedcross and xt’s. The xt are much too padded and bouncy on the heel for proper use. They come with ridiculously padded insoles so that they feel super comfy in the shop before you buy them but for trail running they have no feel other than vague bounciness and little support.
The speedcross are better, less padding all round but still quite high in the heel for me, and I am quite a heavy heel striker. The high heel tends to force my toes into the toe box which is quite narrow leading to toe pain when running downhill. The speedcross have excellent grip in all conditions though imo, and are a decent aggressive trail/fell running shoe for serious gloop. The soles don’t tend to last too long though as they are a pretty soft compound for grip, just like your bike tyres.
If I was a more serious runner i’d prefer something stiffer and more supportive for off road running like the asics fuji ss12