Rules-wise it's pretty easy! since it's a turn based game, with one player it's just always your turn.
Feeling-wise, though, it's definitely a different experience, but the difficulty would depend on the person playing. Since you're not bouncing ideas off of other players throughout play, in my solo playtests the game became much more of an introspective experience where you're trying to come up with ideas that really speak to you in particular.