Why is vibe.d json serializer/deserializer so complex?

[Steven Schveighoffer]

I was looking to report an enhancement request to vibe.data.json (filed 
here: https://github.com/vibe-d/vibe.d/issues/2490), and I started 
looking at the serialization code for vibe. It's really really 
complicated, and I'm just wondering if this is a case of 
overengineering, or if there's a logic behind making this so 
complicated. My iopipejson serializer is super simple comparatively.

Is there a benefit to having all the complexity? I know they support 
both json and bson, but I cannot follow the code very well, and I'm not 
sure what happens where, and which types are responsible for what.

-Steve