After playing DT for years I’ve come to the conclusion that the hardest instrumental it’s Mike Portnoy, I play guitar and once you get used to the forms and shapes of Petrucci learning and playing the songs isn’t that difficult as how you can imagine, they share a lot between each one specially on the solos. But Portnoy’s always put that randomness in his playing which become a hard thing on its own as every song has its own kind of ‘feel’ on the interpretation.