Well to be completely honest, for as much as I praise Unreal4, I've built my own engine once again for my next game -- and one of the main reasons I did was because I do so much procedural stuff in it.

Over the three months it took to do my engine modification to U4, a large part was just spent dissecting their render pipeline. But in its defense, I often found that the engine was built the way it is for good reasons. The unreal editor is vast, even from a user standpoint, and its code is tightly interwoven with the engine core. Despite this, I usually found that the code is as straightforward as it can afford to be, given all the interfaces it needs to provide for services like physics, networking, rendering, editing, etc. So perhaps this is my lack of experience talking, but it's hard for me to imagine anyone making something as comprehensive as U4 without compromising the simplicity of their code somewhat.

On the flip side, I finished working at Monolith recently, and their code appeared to have a much flatter, simplistic structure overall -- but this simplicity actually made the code less readable in my opinion, more prone to bugs, and definitely less functional overall (particularly evident through the engine tools). So there are much worse things out there than UE4 imo.