Crafty.js vs Flax Engine

Easy to learn, based on Entities and Components, the later being class-like objects that entities inherit.

Has open function binding systems, allowing easy creation of custom components.

Can be used for free until you hit a rather high revenue cap, at which point you have to pay royalties.

C# scripting is really well made, comparable to Unity, but without any of the legacy cruft.

Supports most platforms, including Windows, Linux, Android, Nintendo Switch, ...

Has a very modern, performant and beautiful rendering backend.

The entire core is written in C++ and it supports writing game code in C++. The interoperability between C# and C++ is also nothing to scoff at.

Works with any text editor and comes with proper support for Visual Studio, VSCode and Jetbrains Rider.

The documentation is well written, up to date and accepts improvements from the community.

The editor is both powerful and simple. My favourite part is how easy it is to extend the editor with custom magic.

The entire source of the engine is available on Github, though not under an open source license.