It's a bit like someone being amazed that AI research solved chess before they solved object recognition or even walking.
Casting shadows with minimal artifacting in most non-raytraced pipelines is actually very, very hard. Different engines have different solutions to how they hide these issues, but they all have their edge cases (though some are better than others at this).
Likewise, the light leaking you describe sounds like an artifact of Unity's global illumination techniques. Again, this is a very hard problem. Unreal just happens to solve it in a way that works well for your use case.
If Unreal happens to be the right solution for you, then that's great, but it's important to realize that these issues aren't "no brainers".
That one is particularly funny now, as it was made before deep convolutional neural networks beat the ImageNet competition.
I guess my categorising them as no brainers is just that the use case seems so obvious that it not working is surprising. It was just a door casting a shadow onto a floor plane, I may have done something wrong but, that it was even slightly problematic was a red flag to me.
The asset workflow from Blender to substance painter to Unreal also had no hidden demons. It all just worked as expected.
Just to be clear as well, my concerns weren't about the post processing stack which I got working to good effect in Unity. But the lightmapping and material workflow is just a bit clunky in unity. To say nothing of the confusion that having more than one lighting engine causes in docs and online support.