When comparing Li3 vs Lumen, the Slant community recommends Li3 for most people. In the question“What are the best PHP frameworks?” Li3 is ranked 10th while Lumen is ranked 17th. The most important reason people chose Li3 is:
Lithium has unified the relational and non-relational database APIs into a single one. Being one of the few frameworks to do it.
Specs
Ranked in these QuestionsQuestion Ranking
Pros
Pro Unified relational and non-relational database API
Lithium has unified the relational and non-relational database APIs into a single one. Being one of the few frameworks to do it.
Pro Robust plugin architecture
Lithium makes use of PHP namespaces to create a powerful plugin architecture. Almost every component of the framework is replaceable.
Pro Fast Bootstrap & Autoloader
This framework loads faster than most due to its class autoloader. It is flexible as well and handles older classes - but requires the developer to define transforms here for naming convention, includes, etc. Many other frameworks use Composer for autoloading and that significantly slows down their bootstrap time because Composer's autoloader is extremely compatible without much developer interaction (it does not require as much involvement for autoloading legacy libraries). So it's a trade-off, speed vs. a little bit of up-front work.
Pro Integrated unit testing
Lithium comes with integrated unit testing. It also has a test dashboard.
Pro Aspect inspired filter system
Lithium's filter system is based on the paradigm of Aspect-oriented programming which aims to increase modularity by separating cross-cutting concerns and helping speed up development.
Pro Great for building an API
Lithium can return JSON and many other formats from the same actions that render templates increasing productivity in many cases. It handles a "type" key in the route which allows it to render various responses with different Content-Types. This is incredibly useful and easy to extend. This combined with closures in routes makes Lithium a very good framework for building a RESTful API.
Pro Easily upgradable to Laravel
Since it's basically just a minimal version of Laravel, it can be upgraded to a full Laravel app if the need arises. No need for code changes, just import the code to a new Laravel install.
Pro Feature rich

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).
Pro Easy to use
Cons
Con Less than ideal documentation
Documentation is scattered so it takes a little while to learn and figure some things out.
Con Built for smaller tasks like APIs
Con It's based off Laravel and inherits its shortcomings
Lumen inherits many shortcomings of Laravel, such as static proxy classes.
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.
