ENIGMA Development Environment vs FlatRedBall

Almost full support for GML,
The ability to create and access C++ types, templates, and functions, compile DLLs and other C/C++ scripts

Support for Windows, Mac and Linux.

Enigma can support over 90% of gamemaker's GML language

besides the powerful combination between GML and C++, beginners can also use drag and dropping.

Written in C++, many features have been demonstrated running much faster than interpreted equivalents in GameMaker (up to 10-20 times faster than GM 8.1).

Changes are made daily to add new functions/fixing bugs.

A full stack trace with available cores and memory information as well as operating system and Java version including file names and number is generated whenever an exception is encountered, with a handy link to submit the issue to GitHub.

Just check the commit frequency on github :)
https://github.com/vchelaru/FlatRedBall/commits/master

Plus Victor takes community input seriously and is known to shift around priorities based on the pressing needs of the community

Simplifies routine tasks such as adding entities and files to the game, or tuning parameters, via the FRB Editor called Glue

Very active chat on gitter: https://gitter.im/vchelaru/FlatRedBall
Victor (the creator of the engine) is available throughout the day to answer questions and solve any problems that may arise, along with the rest of the community members who are ready to assist in any way they can.

http://flatredball.com/documentation/
Very good documentation not only regarding the API details, but also lots of tutorials covering different aspects of using FRB, either on the code-side or on using any of its tools.