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.
> we found that pushing gameplay systems beyond the prototype stage would require more and more effort
I understand that you're saying that some systems exist that can't work. I'm trying to understand what those systems would look like, and how the user would see it as being different. Do you have an example of the system/mechanic that can't work?
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.