I'm not a learner of Russian (I'm a native speaker), but I disagree with some things from this article.

Alphabet is very easy. It isn't anyhow harder than Latin alphabet. 

What I think is the hardest in Russian - in which part of a language foreigners tend to make mistakes is:

Word stress. Actually, even native speakers make mistakes with it, but of course with more uncommon words than foreigners. It can not just fall at any vowel, but it can also fall at different vowels depending on the case, the word. For example: he has understood - "on pOnial", but she has understood - "ona ponialA". But at the same time: he thought - "on dUmal" and she thought - "ona dUmala". And even the pronunciation depends on the word stress: "On ponial" pronounces the same as it is written: "On pOnial". But "ona poniala" pronounces rather like "anA panialA" (or anA p'nilA...it isn't the right transcription, but it's more close to how it sounds like). 

So, learning Cyrillic alphabet isn't hard - but it's hard to predict how to pronounce a russian word correctly. 

And there are other things in Russian pronunciation which are more or less difficult for foreigners. It's all these soft conconants. Even people who know Russian very well tend to pronounce for example "mienia" (меня) more like... "m'ien'ia" (мьенья). In Russian it's rather a soft consonant + E/A than a hard consonant + IE/IA (hard to explaine!). And of course Ы. 

Cases are hard only at the beginning. More advanced learners usually use it quite correctly! Because it has pretty working rules. 

But verbs aspect... well, it seems to be harder for learners of Russian, because it's maybe something that you need to feel? I don't know but I noticed that foreigners make more mistakes with it.

So, it's what I noticed from my communication with foreigners who learn Russian.

Alphabet? Not so bad.

Word stress? It's not easy, but one can power through it.

Pronouncing ы and soft consonants? Ug. Russian is kinda hard.

Vocabulary? Oof. This is getting difficult now.

Cases? Oh God, what did i get myself into?!?!

Imperfective and perfective aspects? What?? Why??!? This is crazy! 

Unidirectional and multi directional verbs with aspects that change with prefix?!?! WHATTTTTT?!??? Noooooooo!!! WHY RUSSIAN LANGUAGE?? WHY DO YOU HATE ME????