yeah, i actually thought about something similar to what you're talking about after posting but didn't bother editing. i think the main reason a lot of people find the Megaman games hard (and early Castlevania, which you conveniently mentioned) is because they force you to observe more than just blaze through like a lot of other games (i'm mostly speaking of real time and just general non rpg games here for the record). the games require some persistence and for you to slow down a lot more, but i still don't think many parts of them are really that flat out difficult- but i'd rather not get into an argument about the differences between a series just being frustrating and one being truly hard.
i am of course biased because i play basically nothing but nes/snes era platformers these days though, so whatever.
also i just noticed that in my last post that i said both MM1 AND MM6 were the easiest due to bit of a lapse in thought- i'd put 6 easier than the first, for the record.