I think we've all seen the Sequelitis episode
where egoraptor talks about the difficulty in CV1 being a good thing, and how it makes you stop and think rather than just blow through enemies the moment they show up on your screen.
It occurred to me that X6 might have a similar thing going on, this being why it's appreciated by a number of us here on RPM.
There's different kinds of difficulty, good and bad. Bad ones would be making it literally impossible to avoid taking damage, or just increasing enemy stats so you have to wail on them forever without really being different from early game enemies, and I feel that for the most part X6 avoids these.
All in all, it's unforgiving, unlike previous mega games, but fair and satisfying.
I believe Demon's Souls would also be relevant but I have yet to get around to playing any game in that series, so can't say for sure.