I think you're moving the goalposts. Of course Unity can work "just fine" for things like Cuphead. However, DragonRuby is an mRuby wrapper around SDL by the guy

I think you're moving the goalposts. Of course Unity can work "just fine" for things like Cuphead. However, DragonRuby is an mRuby wrapper around SDL by the guy who wrote SDL vs. a 3D engine that was reworked into a 2D engine? Surely Unity has more overhead and DragonRuby can make more optimizations.