While I've never broken a flex cable I've resigned myself to the fact that an iPad with aftermarket adhesive holding the digitizer in place will never hold together as well as the factory glue.
I inform people of this before starting the repair and when I hand it back I have kapton tape holding the digitizer in place externally. I usually recommend they get an otterbox style of case to help hold it and to hide the tape.
It sucks, but it's better than a broken iPad.