I went through 2 Rentec racks like that on my FZS600. I think the box caught in the wind and with the constant vibrations up and down, the racks cracked and split just after the bolts into the fairing. I managed to spot it the first time before it came off while driving. But the second time it went I was out on the road, luckily not with the top box on that day, and the rack bounced down the road and just missed my mate who was following me.
After that I bought a proper GIVI rack.