I will say it this way. Always keep assets modular when you creating them. No harm can be done this way.
The real question is “Should I import modular assets to engine, or merge them in my DDC software and then import”.
You see, the problem is that when you place you mesh manually on level, it is not instanced. Every mesh you place is unique copy. So when you have tons of small/medium meshes which are not culled, they will quickly tank your performance down.
For corridor like enviroments, it doesn’t really matter, you can safely assume that most of the meshes will be culled away.
For open scenes it’s big problem. In the open, there is not much occlusion culling, aside, from puting fog on level, and using distance culling, to cull everything that is farther away from player view.
Instanced static meshes can only be used now in Blueprints, to procedurally generate/composite more complex structures.
So, keep your meshes modular when you create them. Whether you import them modular or composite to editor, depends entirely on use case.
There is an excellent article on the topic of modular design by Bethesda’s Joel Burgess. Although his case study is Skyrim, the points he makes are very general.
Imagine the relative complexity of making a game world the size of Skyrim entirely in Max, as opposed to making kits and having level designers assemble them! Modularity also allows for a fairly straight-forward and effective division of labour between artistically-inclined modellers/texturers and gameplay-savvy level designers.