How hard is learning Serbo-Croatian?
There is no rule what people will find learning the Croatian language and the Serbian language easy and what people will say that is is hard. Serbian and Croatian are Slavic languages, which are known as more complicated languages than world languages such as English, Spanish, Italian, French, German… That’s because the language logic is different. Slavic languages include grammatical cases, and perfective and imperfective verbs.
Those topics are the hardest to learn for students, but not for all of them, especially not for the students from Slavic countries such as Poland, the Czech Republic, Slovakia, Ukraine, Russia, and others, who understand the language rules better than non-Slavic speakers. But Slavs still need to learn Serbo-Croatian grammatical case suffixes because they are not the same as in their languages.
Also, in many cases, there are some words that exist in their languages, but they have a different meaning.
However, this is not a truth that only Slavs will speak Croatian/Serbian fluently, and people of other nations will never be able to. Definitely not true because I had many students, and still I do have classes with some of them who are from the West, started from zero, and today when they talk, they naturally use all the cases, with little mistakes here and there.