Mega Man: Revenge of the Fallen

That depends on your play level. The first time I play through each 'finished' level with all enemies added, etc. I don't do so hot the first 3-4 times. But after 5-6 deaths, I actually do get better and things that 'caught' me last time, no longer do so. So with practice, it'll become as easy as any other MM game you've played over and over again. I can now beat Magma Man's level (with instant death beams and all) with my eyes shut now. I've grown so used to it.

I really can't compare this game to difficulty of any other MM game honestly. The levels are quite massive (the exceptions here are the Intro stage and pretty much all the Wily stages, which are standard MM length). So I can probably guess it would be disheartening to some players to get far in a level to simply lose all their lives and start from the beginning. But hopefully through practice and knowledge of the level's obstacles, the player should be better with time and effort and ultimately persevere. I am trying to make the game hard, but not unreasonable, so hopefully I can achieve that balance.
That and the taking of time to acquire all upgrades and tanks, etc. in the main RM stages will pay off in the Wily stages. For example, every stage of the 8 Wily sections are balls to the wall hard, almost to the point of being unreasonable. My goal is to make it cheap and unfair, but still doable for those with the skills to do it. But if you managed to snag most if not all upgrades spread across the levels, you open up easier, alternate routes through the Wily stages and skip insane parts. Can you do these levels without those upgrades? Sure! But it'd be easier to find the upgrades and simply skirt around the tough bits.