On Unity, after months of work someday you’re going to open the editor and you’ll see this happened to all your game objects scripts:
Soon you’ll realize how hell looks like…
On Unreal 4, you’ll work for months and someday you open the editor just to see something like this:
https://answers.unrealengine.com/questions/288215/100-blueprint-project-corrupted.html
Immediately you know, it’s that hell moment too.
So… I’d say pick the one which works best for you with text based assets and always version control, do as much as you can through code and compare which one has more stable editor tools.
After 6 years engaged with Unity and 2+ working on UE4, I know what is best for me. Find yours :]