jMonkey3 vs O3DE (previously Lumberyard)

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.

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.

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.

Unlike some engines, jMonkey doesn't force its own IDE. You can use its Netbeans-based IDE, but you can also set up a project to work in another IDE such as Eclipse. You can still use the special tools from jMonkey's IDE in such projects.

Because jMonkey is implemented in Java, the same language its apps are typically developed in, developers will have an easier time modifying the engine to their needs.

jMonkey3 handles input from computers and mobile devices. It handles networking, physics, rendering, terrain, and cinematics.

Engine is modifiable.

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.

Modifying shaders can be done either via a visual tool called Shader Nodes or via GLSL that allow you to make your own shaders without the engine getting in the way or having to hack around to do so.

As a proof of how much Amazon is committed to the project.

Advenced stereoscopic reprojection to save rendering time.

Sculpting and painting tools for terrain and instanced details.
Procedural generation of terrain.

Built-in support for roads and rivers to ease the design of rich game environments.

Full source code for Engine Editor and every tool.

Thanks to built-in terrain, time of day, ocean and volumetric fog systems. There is also a gfx pack with various environmental meshes, cloud, particles, materials and skyboxes.

Multiple entity contexts, reflection, serialization, replication, script binding, event bus (EBus) messaging, fully cascading prefabs (slices).
Every reflected property can be fully exposed in the editor with a customizable gui control and can be animated in the built in animation tools.

Online oriented plugins composed by both client, server and administration components to easily add various online capabilities without further work (es leaderboards, login, message of the day, downloadable contents).
Since version 1.7 they has been released at a steady rate.

Very advanced 3D prototyping tool with many modeling features and UV mapping support available. If you want you can build any kind of fully textured static mesh without relying on external tools.

Realistic looking lighting bounces.
It doesn't require long offline precomputation times like other static and dynamic GI solutions based on lightmaps or other similar techniques.

Other than common aws functionality it supports game specific Game Lift for autoscaling game servers.

Modular workflow and flexible cascaded propagation of changes.

To easy add and remove modular functionalities to a project and easy share them.
Modules are called Gems and can contain code and/or assets.

Tessellation and Displacement.
Various Parallax Mapping techniques(POM siluette).
Translucency and Sub surface scattering.
Detail Mapping.
Material Blending.
Emission.
Advanced uv transformation and animation.

Global environmental volumetric fog and/or hand placed custom shaped volumes with featuring full lighting and shadows.

Amazon made available a selection of triple A quality scenes filled with high quality assets.

Implements the most recent technique available (published in 2017) from Anton Kaplanyan.

You can modify shaders while testing the game.

Shader files are modular and annotated to setup the material editing gui and filter out mutually exclusive options. All shader permutation are compiled almost instantly from the asset processor and cached for later use.

You can create games that react to keywords entered in a designated Twitch channel and let streamers to invite targeted viewers into their game sessions on demand with ease.

It is still possible to use the non LTX version with few changes.

Built-in Lua editor with remote debugger to debug Lua scripts running in any device.

Optimized versions of assets can be streamed live to any device connected over the network.
This makes possible to implement asset types that hot reloads reducing drastically the time to test new content and little changes on device.

Multiple grain of control over any kind of engine setting and various kind of Lod systems. Settings can be grouped and applied in batch.

Perforce also comes with a free version for a limited number of team members.

Has skin shaders with subsurface scattering and weighted wrinkle maps, eye shaders with refraction parallax, hair with anysotropic specular reflections, etc.

A must for problematic things like hair , vegetation and scenes with complex solid transparent objects(es glass of water with ice cubes).

Special iteration of Temporal AA to battle the ghosting issue omnipresent in engines using other Temporal AA techniques.

It now works for both static models and animations and support advanced setups(es lods and physics).

Updated renderer pipeline to leverage the full spectrum of precision, luminosity and image processing features of the latest HDR tv.

Animation Spline editor supporting huge range of keyframing and tangent editing tools.

The UI uses the entity component system so it's fully modular and easily expandable. The UI is easily skinnable and supports border scaling images (scale 9). Powerful layouting system supporting multiple screen resolutions. Being based on the entity system every property can be fully animated.

It is elegantly managed in a separate entity context so it doesn't get messed up with other kind of entities and their components like in other engines, but it's fully decoupled communicating with entities in other context on specific event buses.

See here.