public calendar risks

My question is: how secure are iCloud calendars that are made public? How difficult would it be for someone to read it, if I haven't given them the url? I want to add my personal calendar to a SharpTools dashboard that is in my home. But it requires the url of a public calendar.

I know there's been some discussion of how Google public calendars are actually indexed and can be found with a search. When I make my iCloud calendar public, the url has a long (50-100?) string of various characters, which makes me think there is little risk of someone guessing a url. Is there more to it than that? How risky is it to make an iCloud calendar public?