Cloth numbers are great as long as the rider knows how to properly pin them. If they don't, the material is a lot less forgiving than paper or Tyvek held by a pin, which will rip slightly to conform. Cloth wrinkles, making an unreadable mess. BTW we do absolutely score a road race using the rear number. It depends on the size and speed of the group that is passing. The better timers in the area will use two cameras, one over the line and another behind it, aimed at the rear number.