I have the seat and backrest. The backrest has little to no flex that I can detect. It works for me and my passenger. However, I would not advise you to tie any luggage to it. It is held in by a roller pin type device latching to a little slot cut into the support bar. It may come up out of the seat if you hit a bump having the luggage attached to it. All that is needed is an upward pull to remove the backrest.
Other than this, I love the backrest. It is adjustable for forward/backward positions. Being on the short-legged side, I have mine adjusted all the way forward. I have the king seat. Hope this helps.