I am currently experiencing the same problem which is described in this thread: Unity implementation in Swift 3 Project: Crash on app launch - MetadataCache::Initialize(). I was able to successfully integrate a simple unity project to Swift as explained here: https://github.com/blitzagency/ios-unity5.
Problem is that my exported unity project uses a pod itself (GVR-SDK from google). So I installed the missing framework and all compiler errors were gone, but unfortunately my app crashed directly after app start, but I cannot figure out why.
Is there any specific way I need to modify my main project when my unity build also uses pods?
I also got stuck with this issue you are writing about for hours. Simply add -DRUNTIME_IL2CPP=1 to your „Other C Flags“ in your main project build settings and it should work like a charm.