Do you mean spoken or written? It makes a huge difference. Slovene is not difficult to read, but I find it difficult to understand for speakers of other Slavic languages. There was a scene in one film (can't remember which) in which two BCS speakers are having a conversation in Ljubljana, a Slovene comes and talks to one of them (in Slovene) and the other needed a quick (Slovene to BCS) translation.
It's not different enough to be a cryptolect, but far enough to be very difficult to understand for other Slavic speakers.
On the vocabulary:
I find Slovene's vocabulary to be the most Slavic. Obviously there are a lot of internationalisms, but the core vocabulary, words used day-to-day, not technological/scientific/economic jargon, are more likely to be derived from Proto-Slavic or Common Slavic roots.
Having that said, that doesn't mean that the meaning stayed the same or that that purity is necessarily helpful.
For example the fact that the word for bread "kruh" is derived from PS *kruxъ does not make it easier for anyone else expecting "xleb." As a matter of fact, "brót" would be (arguably) easier to understand for other Slavs.
It's not that Slovene uses a synonym, or they have another word that other Slavs don't recognize, it's the use of words, like kruh, that mean something else in other Slavic languages.
Basically, it's false-friends galore.
The fact* that Slovene uses more Slavic root-derived words means that the frequency of creating a false friend is high. If Slovene changes the meaning and other languages keep it, it becomes a false friend. If Slovene keeps the meaning and other languages shift, then it becomes a false friend.
So when people talk about the Communist era and waiting in long lines for "kruh," one must wonder if it's worth the wait. (Kruh is a "crumb" in some languages.)
There are many examples of this, parents talking about their many children/slaves(?), although that's not exclusive to Slovene.
But the more the word gets suffixed/prefixed, the more it moves into unguessable territory.
For example, vitez, vítjaz, víťaz, etc. Hero, knight (warrior), horseman... are not far from each other. Slovak's "winner" is a bit farther, but still guessable (hero, winner, sure). But when I read víťazstvo, the first thoughts to my mind are knighthood (the honor, the title), the territory of a knight, cavalry (an army's mounted division), heroism... Reading something like Víťazstvo lásky (the triumph of love) is very strange.
This confusion doesn't occur (as often) with borrowings as it does with Slavic-root derived words, for example, ritter/reiter-derived ones: rytíř, rícar, rytier, or bogatýrь... Unknown words don't mislead you.
Let's say that strangers are better than false friends.