I personally have one of these. Well the wife did. And now i do. lol. Anyway, there's many a brand to be had. They're all much of a muchness and range from about £16 upwards.
I'd say the obvious applies. Don't point it at the alternator, Battery, Fuse Box or your face. The rest of it will be fine. It's all pipes, harness and solid metal. I've been to the big under car steaming man before too when i had an oil leak. Stuck it on a four post lift, sprayed it with paraffin and then hot washed it off. Zero ill effects at all. If you can go wading in it, i see no harm in a bit of carefully applied steam.
I also hot shampoo and jet wash the underneath of my engine, diffs and gearbox once a month. About a distance of 1m from nozzle. Underneath car looks like it's just left the factory.
I've only done the engine bay once. Not had to do it again. It's still spotless.
Now if there's a leak, i'll instantly see where it's coming from as i know nothing else is leaking. Oh, except my rear engine seal. But at least i can see where it's coming from and it's not that bad to do anything about. (Yet)