It's totally secure, in my experience. I have the LD1100 mounted to the back of a rack (not a Tubus Cosmo, but I don't think that matters) with the Cateye rack/reflector adapter.
Adapter -> rack = solid, obviously. It's held on with screws, after all.
Light -> adapter = solid.
Because my rack slightly overhangs the crossbar the adapter mounts to (a problem you don't have), I had to mount the adapter upside down, as it's impossible to slide the light into it otherwise. Even in this least-secure-of-all-possible arrangements, no problem.