The grammar can be complicated, but with Russian I find the pronunciation can be particularly difficult. There's a lot of vowel I, e, and y sounds that are subtly different. И, й, э, ы, е all sound kinda similar, with ы not having a direct equivalent in English (it's like an "eeyeh" pronounced with the frontal palette which is weird because English doesn't have a lot of frontal letters).
The consonant clusters are also tough because there's so many of them and connecting vowels are absent. A classic example of this is Здравствуйте (Zdrasvuitse) with the Zdr pronounced with a rolling r as well.
I'm sure if I'm trying to say things I've got a heavy gringo accent.
But it’s also simpler in many ways. For example, it doesn’t use the verb “to be”. So, for example, instead of saying “I’m a boy”, you would say “I boy”.
My boyfriend is very good at picking up languages, so he’s been pretty good at picking up Russian too. But yea, he mentioned the unpredictable stress thing being a bitch and that’s something I never noticed.