We've only made good experiences with layered architecture (onion architecture in particular, which is quite straight forward). We also never built it with the idea in mind to ever replace the storage engine. No, the big, big benefit has been that we can test everything without ever mocking a single class.

I also don't get the point about "going back to IoC" ... how is that mutually exclusive from using a layered architecture. This section was weird and didn't offer any good alternative.