There are a lot of technically good projects which fail because of lack of marketing, and a lot of well-marketed project that are too shallow to succeed. Having both is an incredible asset.
That's why I would be extremely excited if they teamed up.
* what platforms can the engine target (pc, mac, linux, consoles, web, mobile, etc
* does it use a typical oop approach with all your stuff deriving from entity? or an ecs? the latter can perform better but is more complex to work with usually
* how easy is it to use? does it have a graphic editor and asset manager? an easy scripting language? decent lighting system built in?
* how well does it perform
* how customizeable is it
* can you edit things while the game is running for quick iteration?