Realistic, real world models are so much appreciated. When you look at HL2, L4D2 etc. it's all so realistic. Nightfire has none.
Back in GoldSrc time the limit of max polygon (model detail) and texture size was very low. So instead of using models which took a lot of memory (for the engine, not your computer hardware) they created most parts of the game with the editor. With Source they increased it big time. So they could use more models (props, buildings, environment etc) in maps. But compare Source engine with Unreal engine, CryEngine even Unity5 and it is very outdated, on the visual part it is.
When you load up your HL2 level, it will only load up a small part of the map. When you go the next part of the map, it will have to load (with loading screen) this part of the map.
With Source games, the engine loads up everything. When you start up, for example de_dust2 in CS, it will load every model, texture, cubemaps, lights, lightmaps etc (another reason why it is limited on things like dynimic lightning or shader options etc).. Everything is limited. HL2 models are heavily optimized, using multiple LOD model (Level of Detail, basicly high polygon to low polygon models of the same model...)
Source engine does support 4K texture resolution, but make every (important) texture in the game 4K and the performance go down big time. Although I haven't tested this myself in Source. Texture size and shaders are a bigger factor in performance than polygons. Plus the fact of how Source everything precompiles and renders. It would result, from my theory, in either massive frame drops (again nothing to do with your PC) or game crashes.
To clarify this load of (bullshit) in normal English.
Yes, "realistic" models is possible in NF:S
No, you can't overuse it + models and textures have to be limited.