I started thinking about strings as enumeration of chars after running into this issue: new[] { "ab" }.Should().Equal("ab") fails.
I know about extending with extension methods, I just try to stick to the defaults as much as possible and avoid writing extra.
I have been using my own set of extension methods for fluent testing, then I found FluentAssertions which appeared to be more comprehensive and readable so I don't want to write my own stuff anymore :)