Self-timer not so easy when your smartphone is your camera. Still do-able, but not-so-easy. And in general, harder to get the "shot that you want", i.e., some notable and recognizeable feature in the background for example.

In any event, while indeed do-able, I just find that what actually happens is that on a long trip I'll end up with at least a few photos with me in them in stretches where I'm hiking with others, and close to none in extended periods when I'm hiking solo.