We almost never hit technical limits in the renderer, streaming systems, etc. Instead, we found that pushing gameplay systems beyond the prototype stage would require more and more effort, as we'd encounter deep engine bugs, or the tooling simply did not cater to our use case.

We ended up implementing more and more tooling outside the engine, and there comes a point where UE4 became a IO/Rendering system. We'd've been happier if the engine were modular in design from the get-go.