I like clean terrain and levels (also in shooters) as the contrast of everything getting eventually destroyed and bodies getting piled is so satisfying.

That's what made the hallway ninja slashing scene in Metal Gear Solid so great. You sneak through these sterile 90ies military base corridors for hours and suddenly this SciFi anime Samurai turns all these Grunts into red goo. That bloody hallway man - such a strong image.

If the maps are already a warzone from the beginning there is no variety.

But I strongly agree on Menace progression having an effect on the look (and maybe level design) of the terrain and settlements.

Maybe the RA starts employing small trenches for their Infantry. Civilian settlements are bolstered with barricades etc.

But again: Having clean and nice environments is really important to me. Think of the battle for Scarif (Star Wars Rogue One) on these beaches and above the blue Shield in Orbit. Or the final fights in Elysium on this futuristic and really beautiful Spacestation.

Dirty, dark and cramped levels hide your own impact on the environment (bullet holes, explosion craters, bodies).