Urethane has some flex and fibreglass dosnt - if the fibreglass patch or repair is extensive and subject to even minor disturbances I figure it will eventually break away at the join/surface connection. I havnt ever tried this Fibreglass+urethane repair remedy, as I simply wouldnt risk even a hairline crack through the paint around the edge of two differing base materials.
The professional approach would suggest that urethane putty is a product to fill minor imperfections, and a urethane welding technique repairs splits and is best to bond patchs of urethane material for a permenant/better solution
