It appears that upgrading the boost libraries regularly block because of some ghost masked package, e.g.:
!!! The following update has been skipped due to unsatisfied dependencies:
selected: (dev-libs/boost-1.62.0-r1:0/1.62.0::gentoo, ebuild scheduled for merge)
skipped: (dev-libs/boost-1.63.0:0/1.63.0::gentoo, ebuild scheduled for merge) (see unsatisfied dependency below)
!!! All ebuilds that could satisfy "=dev-util/boost-build-1.63*" have been masked.