Tileables and vertex painting are the go to, with pipes and metallic sections requiring trim sheets.
Using packed level instances were great as a prefab solution, it allows us to iterate quickly, and changes will cascade down to every instance.
Three Cisterns Sewers Modular Tunnels
My environment work on the ongoing game Iberia: Death Below. These are the tunnel sets used to connect points of interest within the Three Cisterns section of the map. While staying modular, it was a great way to create an art corner and vertically slice while level design continues.