Obliterated two Ultegra Di2 front derailleurs...

Here's what I now know, if you combine the Cannondale house-brand compact chain rings with a Shimano Ultegra Di2 from derailleur you get nothing but hell.  From what the mechanics say it's a matter of the (very light) Cannondale chain rings having a little too much flex.  While the mechanical shifting can easily overcome that chain ring flex the electronic shifting sometimes does it right but other times just slowing pushes against the chain without enough force to move to the big ring.  That slowing starts to bend and tweak the cage.  Eventually you have a weak cage and when I was going on some flat rode and shifted into the small ring I heard a nice pop and the outside portion of the cage was splayed open.  I've destroyed two and am now waiting for stiff FSA chain rings; I hope it solves the problem!