I had a customer that was from the UK who now lives in the US request that I put Rain X on their skylights a few years back. I guess they thought the skylights would stay cleaner longer. Maybe they did. I don’t know. I only clean their windows and sky every 3 years or so.
I have two windows on my house with hydrophobic coatings on them, just to test them out. They definitely stay cleaner for longer, but the process of sealing the glass is time consuming. I think I would only be interested in working it into my business if it was a quick application, like say just adding it to your cleaning solution.