Seems to me, the bit where all these game frameworks get a bit tricky is when you want to target iOS. Apple keeps moving the goalposts (you must use some version of Xcode, or you must target some version of iOS) and so the open-source frameworks have to constantly faff about to support the latest apple things. A few years ago I was building using LibGDX which used RoboVM to target iOS but then Xamarin bought RoboVM and shut it down and etc etc etc...
What Unity offers is a solid way to build cross platform and target iOS. They're able to offer that because they have the resources to keep up with iOS. Open-source alternatives understandably struggle to keep on the iOS treadmill.