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Pros
Cons
Con Inadequate or non-existent doc
Documentation for Lumen is the worst. If it exists, it's probably the wrong version or inadequately covered in the Laravel doc instead. What doc that is there is sparse to the point of useless. You're going to have to love "iterative development" b/c you'll be trying an infinite number of possibilities just to get basic stuff to work.
Con Made to work alongside Laravel
Lumen as a framework is at it's full potential when used alongside it's older brother. Lumen was created to be used for microservices alongside Laravel, which is used for more user-facing applications. If a project is already using another framework other than Laravel, it would be better to use another microframework for microservices instead of Lumen.
Recommendations
Comments
Flagged Pros + Cons
Pro Highest performing PHP micro-Framework
Lumen is benchmarked at 100/rps (Requests Per Second) faster than Slim v3, which used to be considered the fastest and most performant micro-Framework to date with the ability to handle 1800/rps. (1900/rps vs 1800/rps respectively).