I like to imagine Slavic languages as a sort of scale, where Russian is at one end, Polish and Serbian are at the opposite end, and Ukrainian and Belarusian are somewhere in the middle.
No idea about the Hungarian grammar. I heard that Finnish has a lot of cases
Both (and other languages in the family) share one distinctive feature – an excessively large number of noun cases (by Indo-European language family standards).
However, these languages do not have prepositions, i.e. the 16-20 odd noun cases replace them, so it makes it somewhat easier for a new learner.
The noun cases can also be thought of as postpositions despite obviously not being them, but it is a good and simple mental model.
The real outlier is Icelandic, which has a notoriously irregular grammar, multiple noun declension and verb conjugation groups, prepositions and postpositions despite a small number of noun cases.