I have a home parts steam cleaner and use it for detailing parts on the motorcycles.
I find it takes a while to build pressure and have to keep adding water etc = no fun.
Basically you do one small area at a time, very directional and works well.
If I was to try do this under the hood of car, it would take forever.
Here is a quick way using a quality engine degreaser, and this thread tells the story.
LPS Presolve -Under Hood Cleaning!
I cover the alternator, and if you want to cover other main electrical boxes/connectors, have at it, using small plastic wrapped bags. I don't bother. It seems to survive the warm water power washing with no check engine lights or ill effects. Keep in mind with higher pressure washers like the one I was using, I spray at a bit of a distance. Don't go too close to main electrical areas, and never spray the cooling fan motors without covering them up.
My engine bay looks good once this work is done.When I lift the hood on my almost 18 year old ride, everyone says Wow, I cant believe how clean it is under the hood!