Agreed that yarn is faster and reproducibility is critical, but in case you aren't aware, you can have reproducibility with npm too by using the "npm shrinkwrap" command.
Updating a single package version in a yarn.lock file is much easier than updating a single package version in an npm shrinkwrap file, in my experience. With yarn it's just a single command. With npm shrinkwrap, you have install everything from the current snapshot, then install the package you want to update, then run npm prune, then regenerate the shrinkwrap file, then look through hundreds of lines of mostly irrelevant diff to make sure that it did what you wanted it to.
