I have had one complaint in 8 years of power washing houses. I spray the boxes because they are always dirty, and not easy to clean either I might add. I was a CATV and Satellite installer for about 10 years and only one time had a call due to power washing. These companies have standards for enclosures and connections which should be weather proof for a long period of time. It is unlikely that power washing would cause a problem if everything is tightened properly. Most likely the customer has tampered with cable and used non-weather proof fittings, which was always the cause of most trouble calls.
I doubt you were having bad luck, probably just dealt with some people who had the same cable guy or diy jobs. Guys shouldn't be spraying directly into fittings or openings in the boxes.
The last thing I would do is cover plants with plastic. Using a good house wash mix should be safe for most plants.