I only wear my uggs in the winter and think my feet would die if I tried to wear them in 70 degree weather. Even when I go in stores in the winter with them on, my feet sweat.
Agree with PP - they are flat bottom so there is no support at all. I'm willing to bet after a day or two of being on your feet all day and walking, your back will ache. Also agree that they will get soaked if there is any chance of rain.
If you want to change into them at night when only going out for a few hours, that would be do-able, but I wouldn't wear them for all day touring.