Of course, such a situation is difficult for anyone to learn! That’s what I was trying to get at with that Isiwara example above, which shows a language with even less regularity than Czech. The point I was trying to make was that mere ‘declension’ isn’t enough to make a language difficult to learn… you need to actually have some kind of irregularity to ensure that.
It is not as hard for other Slavic speakers, because they are at least used to such a thing, even though their classes are not necessarily the same.