Well, that's a tricky question. Everything depends on what you call a broken image. svgcleaner will reduce the file size according to the SVG spec. So, unless there is a bug in the implementation, your file should be rendered the same. Problems may arise when you use an SVG rendering application that doesn't support the SVG spec correctly... Then you might get a "broken" image. But it will be broken because the rendering application doesn't support correct optimization as performed by svgcleaner, not because svgcleaner broke your image.
Anyway, if you are using a modern browser - everything should be fine.
See Testing notes for details.
No. svgcleaner is intended as a lossless cleaner. It doesn't remove anything that can impact rendering.
Yes. But those elements don't impact rendering.
It's not. svgcleaner could be many times faster, but it's not a current priority.