Another potential problem with clipless is the unnatural twisting motion needed to unclip. Unlike the shoulder joint, there is nothing in the ankles and little in the knees to accommodate that twisting motion. The hips have limited rotation ability, which is limited even more by sitting.
Not good for folks with bum knees. Knees weren't designed for that. Most of us get away with it because our knees are mostly okay. But it may not be right for everyone.
On days when my knees feel a bit achy (usually due to barometric pressure shifts), I'll deliberately pivot from the hip while unclipping, to spread the effort beyond just twisting from the knee.
The amount of float in clipless cleats can be a factor as well. I like my Look Delta cleats with lots of float, but it also requires more twisting to unclip. Fortunately unclipping from Look Delta cleats is smooth, free of friction and responds with a distinct feel and sound. But they're terrible for walking.
Shimano SPD-SL cleats look similar to Look Delta but feel a bit different. There's less float, but clipping in and unclipping feel somewhat less positive, with a bit more friction resistance. Shimano added hard rubber pads to the cleats to make them a little better for walking.
Just little factors that might influence choices of clipless systems. There are many and I know some roadies who prefer mountain bike type clipless systems, and vice versa. Most of my cycling friends prefer Shimano mountain bike cleats/shoes for walking as well.
Another friend switched from SPD-SL to Speedplay Zero but discovered they are absolutely unforgiving of any dirt or debris. Just setting a foot down at a traffic light might pick up a pebble or tiny twig that hinders clipping in again. I'm not sure how much longer he'll stick with that system. On most rides, we're slow-rolling after every stop for about 100 yards while he struggles to clip in, sometimes stopping again to dig out whatever is hindering clipping in.