PU is a manufactured product designed around 50 years ago that mimic the rubber's grip and softness. It is harder than rubber but is superbly durable. The grip on smooth surfaces isn't excellent, but it works well on concrete and all-terrain surfaces.
Polyurethanes are outstandingly able to withstand more loads than rubber because they are harder than rubber and yet more flexible than plastics. Their flexibility is accountable for their strength and remarkable ability to resist impact.