Unreal Engine's deepest limitation is that game simulation still runs on a single core, because single-threaded programming is orders of magnitude easier than multi-threaded.

The biggest limitation that's built up over time is the single-threaded nature of game simulation in Unreal Engine. We run a single-threaded simulation. If you have a 16 core CPU, we're using one core for game simulation and running with the complicated game logic because single-threaded programming is orders of magnitude easier than multi-threaded programming.