The main difficulty with most Slavic languages are the grammatical cases/declensions/etc. German does have conjugations, but they have less forms and there are easily noticeable patterns (at least compared to something like Slovene: https://en.wikipedia.org/wiki/Slovene_verbs#Full_conjugation...). The words might seem scary, but actually require less thinking to use in sentences.
It's not so much the cases but the interaction with cases and gender. I found Finnish easier in some ways, despite the many cases. Because the case endings are always the same (modulo vowel harmony) so you can extract helpful information - something is inside something else say. In Czech a word ending with u has five different possible gender-case combinations (if I counted right) and that's not counting the distinction between short and long u.