Although it sits on its own branch of the IE family, it is still an IE language and not an isolate, so Greek cannot be *that* different from other IE languages. However, I've been taking introductory lessons in Ancient Greek, and in terms of difficulty it seems to be a few steps above most other Semitic and IE languages I've been exposed to.
For one thing, it's not enough to know cases because each case has at least 3 different sets of markers (and then when you read the first line of Homer you find
menin isn't even inflected in the way they taught you because -- hey -- it just sounds better this way

). There are some patterns to these markers but mostly it seems random (compare to the Semitic u/i/a case markers which seem almost childlike by comparison). Oh and the marker -
os can be either nominative or genitive depending on what set of declensions you're using.
Then there are the verbs, where you learn these neat rules for changing tenses (adding augments and endings) but then discover that the verb itself changes beyond recognition so you effectively have to memorize multiple versions of the verb. Now a specialist can discern all the rules and sound changes that lead to these transformations but for the average person I don't see how any pattern can be discerned. Other languages require you to memorize a lot of rules, but they are at least rules, so if you memorize you can predict. Seems harder than that for Greek verbs, but then again I am an absolute novice so I may be missing something.
That said, Latin seems to have most of the same issues (five declensions instead of three) though I hear it is a bit more regular. And I've heard of languages with even more complex case and inflection systems in eastern Europe and the Caucuses (the latter would not be considered IE however). And Modern Greek is apparently a much more typical European language anyway. So, difficult, yes, and somewhat irregular, but can't say it's unique.