Unless you are going to tour extensively "back of beyond", I wouldn't worry about bottom bracket failure. In the unlikely event of failure, any half decent bike shop will have Shimano Hollowtech II replacement bottom brackets at reasonable cost and installation is a matter of a few minutes. Don't use the GPX experience as a guide to external bottom brackets in general. Shimano's, particularly 105 level and above, are fine.
Excuse the question, but how does one affect chainline with ext bb's?
The chainline on HTII cranks is defined by the spindle built into the drive-side crank arm and is fixed. The bottom bracket is not a factor.