Depends. They have six cases, which is standard for a lot of indogermanic languages, and their declension is mostly consistent. I never learned German as L2, but I imagine the fact that in German cases are not clearly marked on the noun but by the combination of article and noun and that we use two different but very similar marking systems depending on context as utter nightmarish for L2 learners.

Furthermore it’s the only European language there is. Every other language spoken in Europe descends from the Eurasian steppe. Well, most likely with a pinch of Kaukasian. It’s several millennia overdue that we honour the Euskari!