It comes down to language structure and how similar they are. For example it’s easier for me to understand Dutch than Danish. Luxembourgish (recently recognized as separate language) to me is rural Hessian with some random French words thrown in - again easier than Dutch to me. For the German dialects you can use the same writing and it should be understood throughout Germany, Austria and most of Switzerland. Except for regionally used words like e.g. “leiwand” (Viennese for good/perfect). Another exception is if the writer is from Cologne - nobody understands their language.