Will Ghost of Tsushima be as hard as Sekiro?
Ghost of Tsushima will not be as hard as Sekiro, nor any of the Souls games. This is because a Souls game's main premise is "sense of achievement through overcoming an extreme challenge." That challenge takes precedence over everything else, as it is directly proportional to the elation or joy the player feels when he overcomes the obstacle.
This does not mean though that Ghost of Tsushima will not be engaging or challenging or skill-based. It's just that they are different in how they view the concept of difficulty in gameplay.
Sekiro does not, and will not, compromise on the challenge that it throws at you. You either learn, adopt, and succeed, or you will end up stuck and walled by a boss forever. You will miss more than half of the game's content if this happens.
Ghost of Tsushima is more accessible in this context, as the gameplay challenge is not its main core.