This article is spot on in terms of the reasons why I moved all our dependencies at Sparta to Yarn as soon as it was released:
Yarn addresses issues such as indeterminate dependencies, network issues/npmjs being down, and parallel downloads in order to provide more value over npm.
Source: https://circleci.com/blog/why-are-developers-moving-to-yarn
UPDATE:
NPM 5 brought a ton of improvements and they seem to have fixed a lot of things.