Personally, I took to Unreal very fast. I played with it for a little over a week before deciding to throw out our 8-month old Unity version and start over in Unreal. Maybe because it’s an older engine and I’m, well… kinda old, but the way Unreal is put together just makes a lot more sense to my brain. And when it doesn’t make sense, I can go look at the engine code and figure out where the disconnect is.
I’m surprised Unreal devs prototype in Unity, though. I find the iteration speeds to be much faster in Unreal. When we first switched, Unity might have been a little faster because UE4 Mac support was a little rough back then, but now, it would take me so much longer to do anything in Unity. Another team here is working on a Unity project and I’ll periodically help debug or profile, and everything just seems like it takes so… much… longer. Part of that is because we’re mostly a mobile shop, and I can go right to device from UE4, but have to output an Xcode project from Unity and then build to device.