Ranked in these QuestionsQuestion Ranking
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 Integrated unit testing
Lithium comes with integrated unit testing. It also has a test dashboard.
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 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 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.
Con Less than ideal documentation
Documentation is scattered so it takes a little while to learn and figure some things out.