Yarn and npm was discussed in-depth on JS Party #29. Spencer writes on the Mixmax blog:
We tested that this flow with npm 6 would work for our needs and we suggest you do too. If you need the absolute fastest package manager, then you may still find Yarn to be best. But if you’re looking to simplify your setup, we’ve found that npm 6 recaptures a critical balance between speed and reliability.
Spencer and team also shared deyarn a command-line tool for converting your projects from Yarn to npm.