It really isn't easy. Believe it when people say it's hard. It really is. Recall Battletoads and the original Contra? Around that level of hardness.
I wouldn't go
that far. You don't have a limited number of lives, there are sufficient shortcuts and checkpoints, and you're not required to memorize the pattern and placement of hundreds of enemies at once.
Dark Souls is tough, but perfectly beatable, even in areas that weren't as playtested as much as they should have been, unlike [censored] Battletoads.
I can hardly call old NES games hard, they all follow some formulae with tight timing and resource control, just practice more and you should be able to handle them with no problem. It's all about practice, even the 5min Mario Brothers run, it's all about practicing.
Then what the hell constitutes a hard game?