I bought a PB Blaze 1/2W & SuperFlash Light Set the first of March this year the same day I took deliver of my new Trek 520. I mounted the SuperFlash directly to the rear rack in the slot provided. I commute about 100 miles a week so in 5 months I have well over 1800 miles with the rear light mounted in that position.
A week ago I reached down to turn off the light before boarding the train and it fell off in my hand. I hope you can see from the photo that the top edge of the molded frame on the back of the light that accepts the removable clip had failed. I emailed PB customer service and had a new back half and clip in under a week. A simple replacement for free.
I have mounted the rebuilt SuperFlash to the rear light strap that came on my Topeak Rack bag in hopes of the prolonging any furture failure. I think the daily pounding on the direct rack mount was to much. It wouldn't take PB much to redesign a thicker piece across the top of the light.

How tight was the SF on the rack? If it rattled around, that could have put stress on the clip. It seems like the best mountings are totally compliant (like on the loop of a bag) or totally rigid (using the bracket that comes with the light), and not just for SF, but for all lights.

The Bontrager rack has the slot so it didn't make sense at the time to use the mount that came with the light. I just slide the clip onto the light and clipped it in place. It was loose and I am sure the bouncing around and being uncushioned attributed to the failure. But, that said, the design is still pretty weak across the top.

No, I won't argue that.

I also don't expect that to change since this design is going by the wayside in favor of more power lights. In fact, YoSpiff recently bought some look-a-likes that look identical except they don't say Planet Bike SuperFlash. I suspect they're already selling the tooling.

You can also try a white zip tie or two, to secure it in place.

I can't for the life of me imagine how zip ties would help this design problem.
I'm going to fire up the hot glue gun and run a bead across the top to reinforce that point. Might even glue the clip in place to add more strength.

Not to put words in Digital_Cowboy's mouth, but I suspect he meant wrapping the zip ties around the body of the light and then around some suitable place on the rack, to take some of the stress off the plastic mount. As you say, lights mounted on rear racks take a massive beating due to the shock from the rear wheel being transmitted directly up to the rack, due to the rack being bolted (usually) directly to the rear dropout. I have lost several this way, of various brands. I did lose PB once, but ironically, it wasn't the mount that broke, but the whole front half of the light, which holds all the electronics and batteries, came off and all I was left with was the mount and the back of the shell. Zip ties would keep the two halves together, and also spread the load out and keep the light from flexing around its attachment point. Unfortunately they will also probably block some of the light, so not a perfect solution.