The Lancaster had a greater tendency to break up in mid air than the better built Halifax.
What's your source for that? By most accounts the Lancaster was the better built; partly due to the Manchester being designed to cope with a catapult assisted take off.
The end result of the overbuilt nature was the ability to carry the 22,000lb Grand Slam.
There's a few well known losses due to issues with wingtips and the tailplane, but they were ironed out fairly quickly and not half as destructive as the whole series of Halifax that had uncontrollable rudder issues.