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Pros
Pro Easier to maintain scaffolding through code
Because Slush is built on Gulp which puts code before configuration, scaffolding can be done in a simpler more straightforward way. Creating scaffolding through configuration requires more knowledge about the scaffold's specific implementation and remembering documentation, whereas doing it in code is more self explanatory and requires less domain knowledge to get others up to speed.
Pro Scaffolds are easier to hook up together
Because all Slush scaffolds share the commonality of streams, it's easier to combine them through that common interface. With configuration based workflow wrappers, different components may have specific implementations that can cause problems with interoperability. With streams, there's better encapsulation of functionality, making it harder to mess up a scaffold leading to better compatibility.