When comparing Fly vs Yarn, the Slant community recommends Yarn for most people. In the question“What are the best Node.js build systems / task runners?” Yarn is ranked 4th while Fly is ranked 13th. The most important reason people chose Yarn is:
One of the most important aspects of Yarn is determinism (predictability). The lock file ensures that the same dependencies will always be installed in the same way and order regardless of the machine for a given repository.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Takes advantage of ES6 features
Fly is written to use new ES6 features like generators and promises.
Pro Support for concurrent tasks
Fly supports parallel processing and concurrent tasks via Fly.prototype.start([tasks], { parallel: true })
Pro Smaller build files, with same benefits of a huge one
If it's a fly-
plugin you don't have to require it like you do with every other build tool. Theres no need for wrapper functions for each plugin (aka .pipe()
). If it's a fly-
plugin you just call it directly. The build file get's to the point quickly without adding all the extra fluff. This makes your build file very clean and easy to read!
Pro Very easy to setup
Fly is easy to setup, there are no caveats that you have to know since everything is very intuitive. Also with fly you don't have to repeat your self several times. You just define out what you want and that's it. If it's a fly plugin it will be required automatically without the need for an additional plugin.
Pro Fly plugins are required automatically
If you want to use a Fly plugin in a flyfile, it's automatically required
without needing an additional plugin.
Pro Can run subtasks inside a task
Fly supports running multiple subtasks in a single task without having to create a new one.

Pro The same results will be yielded every time yarn is run in a repository
One of the most important aspects of Yarn is determinism (predictability). The lock file ensures that the same dependencies will always be installed in the same way and order regardless of the machine for a given repository.
Pro Can tell you why a package was installed
yarn why <query>
can tell you why a package was installed and what other packages depend on it.

Pro Good network performance
Yarn efficiently queues up requests and avoids request waterfalls in order to maximize network utilization.

Pro Offline mode
If you've installed a package before, you can install it again without any internet connection.

Pro Flat Mode
Resolve mismatching versions of dependencies to a single version to avoid creating duplicates.

Pro Multiple registries
Install any package from either npm or Bower and keep your package workflow the same.

Pro Network resilience
A single request failing won't cause an install to fail. Requests are retried upon failure.

Pro Good documentation
It looks like it has good documentation.
Pro Security is put at the forefront
Even though it's still in its early stages of development, security is one of the core values on which Yarn is built. It uses checksums to verify the integrity of every package before executing its code. This also helps avoiding errors related to faulty caching or captive portals.
Further steps are also being taken to improve the security of Yarn which will be implemented in the future.
Pro Built by the community for the community
Even though it's backed by Facebook, Yarn is built as a community project first and foremost. It's completely open source and hosted on Github. It's released under a standard open source client and has its own GitHub organization and set up to work under the same governance model that other successful projects have used in the past, such as Rust and Ember.
All of this means that both existing and new contributors will always work together to improve the product and introduce new features while also keeping in mind suggestions coming from the community.

Pro Backed by Facebook and Google
Was created in a collaboration of Facebook with Exponent, Google and Tilde.
Cons
Con Limited number of plugins
There's still a very limited number of Fly plugins. This can contribute negatively in development time and energies spent to make a plugin to complete a task for which there's none yet.
Con Takes up disk space
Yarn adds to your disk space usage since it stores dependencies locally.
