I've washed out my engine bays with a soap soak and garden hose pressure water before without many issues. The only issue I did have is I found out that I had a crack in my knock-sensor and got a knock sensor CEL after washing it. I wouldn't suggest doing it to a hot engine, though after you do wash it I would take it for a long drive to dry out the engine bay, used some compressed air to blow it off first if you can.
That all being said, I have blasted my hot engine at the car wash in the Winter, but keep the high pressure wand as far away as you can, and keep it moving. If you hold the wand a few feet away the water won't have enough force to work past any seals or damage any gaskets. Also try not to blast water directly on any electrical connections, if you do and you are worried just pop them appart and dry them with some compressed air, but they are usually pretty well sealed.