I have cycled extensively for the past three decades. Three years ago I began riding in flip flops exclusively. The relaxed lifestyle is perfect for me, however the chill look comes with risk.
Risks of riding with flip flops casual shoes:
1. The soles don't have much traction, so they can slip off the pedals causing a loss of control. Replacing the stock pedals with a better gripping version mitigates the risk.
2. Foot protection if the rider drops a foot sideways. I am not certain how to mitigate this risk.
3. Accident risk. I do believe I have less control of the bike with flip flops. However, riders treat me differently on the road, I believe they are more aware of my presence.
Altogether, I don't think flip flops are for everyone. However, I smile more, feel more comfortable, and develop my feet better ( shoe support minimizes foot development ).