Not FUD. Backup: take a look at the number of outside contributors in Unreal engine release notes for each release. Its not FOSS, but still commercial licensed

Not FUD. Backup: take a look at the number of outside contributors in Unreal engine release notes for each release. Its not FOSS, but still commercial licensed visible source. People can find and fix the bugs they encounter. Then take a look at commercial middleware vendors that can integrate with Unreal on their own, without having to involve Epic. Take a look at the list of companies committing development time to Blender because the see their effort improving the ecosystem instead of just adding to Unity own IP. Also take a look at the ancient .NET integration in Unity that has been strangled by licensing problems with Xamarin. Even though Microsoft has re-licensed .NET recently, it doesn't look like Unity will have an upgrade anytime soon. The only development environment for Unity is C#. Look at the numerous VMs and language integration in Unreal, in addition to any language that links with C++. Answering your own questions is obnoxious. Public API documentation for software only available behind a commercial subscription is completely irrelevant when integrating with open source.