I have had one motor shaft break, and it was due to a misalignment with the lead screw. Some of the motor shafts are two pieces that appear to be joined with electric current passed through the shaft pieces. Mine has smoothly faced square ends with three small pits where the faces were attached. It was not a fully welded joint. I don't know if this is done as a safety feature or not.
My solution was to drill the mounting plate holes larger so the motor is better aligned to the lead screw before tightening the motor mounting screws. Shaft alignment needs to be as close to perfect as it can be, and don't depend on the coupler to bend enough for alignment. It just puts the shafts in a permanent bind.
CarveOne
http://www.carveonecncwoodcraft.com