Sandals are fine when the land is flat. Had someone bring some skookum hiking sandals (just about as heavy as trail-runners) on a 12 day trip in the Colorado Rockies and they were slowed down quite a bit while sidehilling as there was less support from the sandal than even the lightest weight hiker. Also, they slipped worse on the snowfields we passed.

Sandals might be fine for flat or semi-open desert or rock trails but they don't fly so fast on anything with varying slopes or pass the slippery test very well. I'm doing the North Coast Trail next month and despite the mud problems, they would be not that great balancing on slippery rocks even when beach hopping and the sand abrasion on the beach would make them useless for any distance on sand. I used Chacos in Mexico for some short hiking (up to 5kms) but wouldn't consider it around where I live. You can make way better time in shoes, and even Keen's don't do slopes or slippery very well.