Over portage? Installation time, outcome repeatability (how many binary packages do you have installed?) and ability to reliably roll back to previous state.
If 'installation time' is touted as improved, then zero compilation is implied. That means lost portability and binary packages, which is unacceptable to many.
As for repeatability, anything that touches the network is by definition unreliable. Portage offers package caching.
To make portage operations repeatable it is necessary to snapshot the system, kernel and portage tree, run emerge --fetchonly <package-spec> then provide the resulting /usr/portage/distfiles/ as part of the environment seeking to repeat the process. It's achievable, but non-trivial. Gentoo-family distribution developers do not optimize for this use case but it does work well.
Personally I have created an alternative to the other options in this space (thoughts at http://stani.sh/walter/pfcts) and am attempting full cross-platform support (any OS, any build system or deployment process).
