I usually don't understand broad sweeping blanket statements like this. Anything can be criticized for anything in a specific context.
Your example for instance is Pokemon. Pokemon isn't criticized for being easy, it's criticized for never posing a challenge ever. No one expects Pokemon to be hard, it's fanbase is too broad between the ages of 7-40 for them to make the games difficult. People just don't like that there is rarely ever a challenge in the games at all.
That being said I do like how in Pokemon the challenge is more so catching the creatures than fighting them, but i'd still like the occasional difficult battle.
Dark Souls is beloved for it's consistent difficulty because it makes every fight, every moment to moment gameplay second, memorable. It's much easier to consider a game unmemorable if most of your time is spent tackling a consistently easy challenge. At the same time, a game can become less memorable if it's just difficult every second, which is why I think I prefer Dark Souls over it's successors.
