<blockquote>...the easiest way to avoid "errors" is to say the strings are ISO-8859-1</blockquote>
That's the real core of the problem.
The scary myths about broken UTF-8 are very likely being spread by precisely the same people who broke the UTF-8 in the first place because they cannot imagine anything beyond ISO-8859-1.
I live outside US and Western Europe. If I do something as sloppy as treating UTF-8 text as ISO-8859-1 (removing 8bit chars, escaping, whatever) I completely mangle it. So, people don't do it. I meet broken UTF-8 rarely and when I do it invariably comes from the western countries.