I appreciate the way they went about it. They switched engines back in October but didn't say anything about it until they had something to show for it. An engine switch is a big prospect-- best to announce it when you can show that you're up to the task!
See the info about it on their Kickstarter news article: https://www.kickstarter.com/projects/1598858095/system-shock/posts/1820162
And if you want a comparison of what could have been in Unity vs. what will come with Unreal, check the main video here: https://www.kickstarter.com/projects/1598858095/system-shock/