I came across this interesting article on Bike Radar today.
Shimano EP8 motor bottom bracket spindle failures caused by "manufacturing defects", ethirteen claims
e13 have used independent analysis to confirm that it is a design defect in the spindle and not caused by e13cranks. They shared this with Shimano who said that they were aware of 49 spindle failures and not one was associated with a Shimano crank. Shimano stated that only Shimano cranks were tested and approved for the EP8 motor.
Also, that if you have an EP8 spindle failure with a non-Shimano crank and/or have a non-Shimano crank on your unfailed EP8 spindle and are concerned, then contact the bike manufacturer. My understanding from the Bike Radar article is that Shimano are saying it is nothing to do with them.
Bike Radar also chased up some of the manufacturers that use non-Shimano cranks on EP8 motors. At the time of going to press only Commencal had replied. My reading of their response was that they were doing everything that you and I would hope. That includes replacing your e13 cranks for Shimano ones, even if you have not had a failure. Go Commencal!