Many NES games were hard, no doubt about it, and basically the only way to master them was to memorize the "difficult" spots. Many of the games were difficult not because of bad controls or particular bosses with nasty attacks or because you needed god-life reflexes, but because of the overall level design, enemy placement and enemy spawning. Many NES games were also quite notoriousfor utilizing the "constantly respawning enemies" syndrome where the game would immediately respawn enemies you already battled before just because you made the mistake of moving backwards enough that the screen scrolled. Of course level design usually walked hand in hand with this respawn syndrome where enemy placement would be such that they appear immediately after you've made a jump or some other necessary and potentially risky move (Ninja Gaiden being the most notable and infamous, but certainly not the only one - Megaman for example, is equally guilty). You don't see this kind of thing anymore in modern games, and much of the annoyance comes from noticeably bad gameplay or design choices, or piss poor enemy AI, etc.