One thing people miss about Flat pedals is the ability to slightly vary your foot position. Slight shifts in foot position can help with cramping or tired muscles. Can't do that with clips.
I trained for Ultra. I’m looking for comfort and speed, I have a touring binding with one side flat and the other side clip-less. The clip-less is not that tight so I can change me foot position slightly and the flat side is for cautious riding like the city or very busy circulation of people or car, etc,,, I have a wide range of foot and leg 🦵 position.