In addition to noun inflection, verb aspect, pronunciation stress, and punctuation trouble many native English speakers. That's in addition to all the simple irregularities, like irregular nouns and verbs.

Stress even troubles native speakers. When I lived there, I saw slideshow "where 's the stress?" quizzes used to fill time on screens in taxi buses, waiting rooms, and the like.

That's saying that getting to the lunar orbit is the only difficult part in landing on the Moon. The whole complexity of inflectional languages is in the inflections. It's also why Slavic (or Turkic) languages form such a large continuum of mutually almost-intelligible languages.

Compared to inflections, everything else in Russian is simple. The word formation using prefixes and suffixes is weird, but it's not like English is a stranger to this (e.g. "make out", what does it mean?). The writing system is phonetic with just a handful of rules for reading (writing is a different matter).

It's a little bit like moving from Italian/French/Spanish to English, except that English has some tenses with no direct equivalent in those languages and a ton of phrasal verbs to learn, but that's vocabulary and not grammar.