Welcome, rbl!
I'm sure there are others who have run their Mirages/Space Stars through auto washes with underbody sprays. (I added a few words to your thread title to maybe get more eyeballs on it -- hope you don't mind.)
I seriously doubt it's a problem. To harm the engine, the wash would have to : 1) somehow get water into electrical components wet that aren't well sealed, or, 2) somehow get water in the engine intake. I can't see it happening.
When you say you read of other people having problems, do you mean with Mirages?
I've always power washed the entire underbody of my cars a few times a year, and personally have never had problems.
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View my fuel log 2014 Mirage ES 1.2 manual: 63.2 mpg (US) ... 26.9 km/L ... 3.7 L/100 km ... 75.9 mpg (Imp)