Ranked in these QuestionsQuestion Ranking
Pro Free and open-source
jMonkey is completely free, meaning it's possible to develop and release a game with no fees or royalties. Because it is open-source, jMonkey has plenty of people fixing bugs and, adding to the engine as well as creating a variety of plugins that can be used in the engine.
Pro Offers both low-level and high-level ways of editing shaders
Modifying shaders can be done either via a visual tool called Shader Nodes or via GLSL that allows making own shaders without the engine getting in the way or having to hack around to do so.
Pro Not limited to using its own IDE
Unlike some engines, jMonkey doesn't force its own IDE. It allows setting it up for use with other IDE's such as NetBeans or Eclipse.
Pro Ease of extensibility
Engine is modifiable.
Pro Engine modifications can be made using Java, the same language jMonkey3 implements
Because jMonkey is written in the same language it implements, Java developers will have an easier time modifying the engine to their needs.
Pro Java is a great development platform
Java is a well-optimized just-in-time compiled language. It's faster than languages without an effective native-code compiler such as Python or Ruby, similar in speed to other just-in-time compiled languages such as C#, while slightly slower than compiled languages such as C or C++ (with some low-level and numeric benchmarks being similar to C++).
Java also has a wide variety of high-class IDEs available.
Pro Freedom of choice for architecture
The user is not compelled to use any programming architecture nor standard in order to make a project working. JME allows the freedom to use what is best for a game.
Pro Multiplatform support
Code can be ported to mobile (iOS is in the works) and other Android supported devices with minor changes to the code (just change some implementations that vary on the platform such as inputs and user interface). It can even run on certain Raspberry Pi devices.
Con Slow release cycle
jMonkey3 lacks manpower to have a fast and decent release cycle.
Con Scene composer lacks ragdolls
Con Not an engine for total beginners
While it's clear that you need to know Java first before using this engine, it is recommended that you have some programming experience as well. Most performance issues and memory leaks are more due to bad programming practices than the engine itself.