It is a good question, I'm glad you asked it. I would think metal shoes would be much more slippery.
Just FYI Kitten, ours is a black asphalt road. We ride out of the ditch and up onto the pavement, sometimes at a fast lope. We've never taken a slippery step.
"Until one has loved an animal, part of one's soul remains unawakened..."
- Anatole France