What I think is most difficult about Gwent is not a particular faction, but the three round mechanics: when to pass, when to push, subtleties of reach and tempo.
Playing all factions ( well, possibly not SY — I’ve had zero interest in playing that because it hinges around cards I hate and an ungrateful mechanic) is relatively straightforward; and deck building has been rendered moot by a horribly narrow meta and the ready availability of net decks.
For me at least, the hardest parts of playing factions is getting the interface to put the cards where I intend, noticing statuses (especially shield, defender, and veiled where the symbol often doesn’t show up clearly against the card art), and remembering the details of cards whose effects change depending upon the row in which they are deployed.