ngBindHtml does not work if SafeHtml types don't have a toString function

[mprobst]

If you have a $sce service whose objects do not return their value in toString, ngBindHtml will never receive updates because it relies on the toString value for change detection. That's pretty common because toString by default just returns [object Object], and for optimized binaries your optimizer might strip a toString method.

Using toString in ngBindHtml is overall a bit hacky, it runs counter to AngularJS' normal change detection. The fix is to just use a deep watch.