Honestly, Smash has always had very weak single player modes. For someone who has dedicated a lot of hours in Melee and Brawl trying to 100% everything because I had nothing better to do at the time, it was still a little frustrating. I managed to clear Classic, Adventure, and All Star with all characters in Melee, and then Brawl came out, and In Brawl, I just gave up, since it wasn't fun anymore.
Which is why I am glad that things such as Classic can be done with a friend now, since higher difficulties are more stressful than fun. At least with a second player, you have greater odds, and the added bonus of the fun factor of working together.
But for anyone saying that any Smash game had a fun single player experience? Not really. It was fun when you first experienced the modes, but if you are like me, who has played just about every Smash, single player just becomes less and less fun with each new installment. Mainly because it was never good to begin with. It was just that I was younger, at the time, and nothing better to do.
(The only thing I unlock in Smash games are just the characters, stages, and music.)