Is the exam hard? I would answer the question like this: Anyone who fails the DSB-TÜV exam after 4-5 days in a data protection officer (Datenschutzbeauftragter) course either really has very bad luck or simply was not paying attention at all during those days.
From the sheer volume alone, you cannot compare it to the tough exams like CISSP, CISM & Co. Anyone who holds a CISSP, CISM or TISP will certainly pass the certified data protection officer DSB-TÜV exam.