So if you're in space, and you're flying nearby a big ship full of triangles, such as an imperial star destroyer, you run your framerate down to the ground if you render even only 5% of it, as it renders everything else as well.
You can mitigate that to some degree by breaking up objects, but even the shell being plain would still cause massive issues.
For first person, open world, or large environment type game, I'd recommend Unreal. For 3D isometric style games, it's perfectly suitable with knowing the caveats and minimal adjustments from an asset creation perspective.
