Comment by vvanders

As someone who's worked professionally with these engines I'd largely agree.

The other thing to keep in mind is that engines are build towards certain strengths/game-types. Trying to do open-world in UE3 required making some fairly invasive and substantial changes as the engine was built more for somewhat on-rails shooter. It didn't support geometry streaming and a number of other things that are needed to seamlessly transition between large open areas. I remember a number of MMO-style games used UE3 purely as a rendering front-end and mostly built the game tick, physics and netcode back up from scratch.

There are "tells" for engines if they aren't tuned properly(i.e. it was really easy to tell UE3 by the 30s frame hitch as the periodic GC ran) but a fair bit of that can be addressed if the team wants to put in the time to enhance or rework the systems involved.