It took me several days (and nights) to clean the underside of my 66 GT Fastback. There was nothing in the way, no drive train, no plumbing, no cables, which made it easier (yeah, right). On your back, on a creeper, crud falling in your face, hands wrinkled from solvents, sneezing from some nasty grease remover, a fun time.
I haven't had to use a steam cleaner service for a while, but the last time I did the car would not start for a while - too much moisture in the engine compartment I guess. That may be why I am reluctant to do use steam again.
Jim