When not including errors introduced by wrong input, over 90% of all submitted XML files are succesfully converted to CSV, Excel and Json. The converters are designed to be super easy to use and therefore lack some advanced features. These advanced features, which you may find in some commercially available products, you to fine-tune the conversion results. We are contemplating such a commercial product (without doing consessions to the free online conversion tools of course!). See also our statistics page.
It depends on the type of conversion you want to perform. The points under are mostly focussed on errors which you may encounter during XML to CSV and XML to Excel conversion.