I hope you aren't too offended by this, but I chuckled a little bit while reading this because it reminded me how some of the hardest problems in a domain (like light propagation/global illumination/shadows) can seem trivial compared to things that are actually rather easy (most post processing effects).

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".