Is Dutch harder than German???

It's not only words, but the grammar, syntax etc., in other words, how all of those words fit together to form a coherent phrase or sentence in the language.

The study on the ease of learning 15 European language didn't consider Basque, Estonian, Hungarian or Gaelic. Possibly its consideration of Finnish was less than complete. There are lot of factors that need to be considered.

English with a Phonetic Alphabet, say the the Shavian Phonetic Alphabet,
would probably be much easier to learn than German and most of the Romance Languages. I would not be the hardest, for sure.

Regards, Paul V.