...Of course, even a "naked" engine room is likely to have a neat, shirtsleeves/whitecollar control center somewhere. And most of the time, Scotty would be working in that section of the engine room, not in the grease pits or axle shafts or coolant waterslides. Essentially, something would have to go very badly wrong if Scotty had to leave his comfortable control center.
Granted that something often did even in TOS. But it's more realistic for a single person to repair things in a single room than in a vast maze of machinery; if something was really broken in the deep bowels of the engines, it wouldn't be logical that Scotty, working almost or completely alone, could repair this within the time allocated by the drama. It's more logical to think that it was some central control doodads that broke in those episodes, and that Scotty was able to reroute rather than repair - which is something achieved better at the small, neat control panel room than in said deep bowels.