Some of the chemicals they use to clean the engine bay are caustic. You can do it a few times, but as the car gets older you are going to see more issues if you do it. I have seen gaskets go, alternators go and hoses go - I can't say the it was the cleaning but in each case it happened within a week.

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07 X5 E70 4.8i
04 Z4 E85 3.0