It is very much a matter of taste. I think, because I lived through this era, that I actually like harder games. So the difficulty of say, Metroid makes Super Metroid seem better to most, but I actually like Metroid better.
Well it wasn't the difference in difficulty that made me like Super Metroid more, but I like how you re-tread old ground in the sequel better than the first, on top of all the fun abilities you could use like the Speed Booster.
Though on the subject of NES games being more difficult, I do think that's generally true. But also, many of those games were more difficult because they were designed like arcade games, with no consideration for the players time in mind.
As in, no way to save your progress or continue, after a few deaths. For arcade games, that made sense because they're designed to make the player insert more coins to continue.
Not every NES cartridge had the capability of saving data though, but some games used a password system instead to let players get back to the stage they last reached. (Though this could also be used to skip stages if you wanted to cheat and knew the code.)
Anyway, I feel like my favorite games are not particularly difficult. And some have even been too easy. So I don't think the difficulty is one of the more important factors when it comes to my overall enjoyment of a game.
I've played games that are difficult by design, like Nioh. And the difficulty itself was enjoyable. But it didn't make the overall game much better.
And it was a good game. Just not a game I consider one of my favorites or anything like that. On the other hand, I found Nier: Automata to be too easy (on Normal) and yet it is one of my favorites.
Also some of these NES games relied on things like an instruction manual or other inserts to be playable (like Zelda). If you try playing the game now without these tools it doesn't feel quite right.
Yeah, Metal Gear Solid on PS1 did that as well, with a codec number you had to look up on the backside of the box of the game.
But we actually didn't have the game box, so we thought we were supposed to look at some item in-game.
So, I actually do like the NES games better in probably more instances than you, but also, at it's time it was far more amazing than any other console in its respective time.
Master System had Sonic, Alex Kid in Wonderland, and some other cool games as well, but I preferred the NES library.
For me Mario 3, Zelda, Metroid, Mega Man 2 and Punch Out were the best in the series. The other franchises I mentioned have better entries on later consoles. But even considering all that. It would be nice to have a modern console with this level of originality.
Mario 3 is actually another rare example where I liked the NES version better than the SNES Mario game.
But they're strange games in that while I was playing them, I really enjoyed them. But when I now consider replaying either one of them, I get the feeling I dislike both of those games.