As I have been playing SSBC, I have found it to be a very excellent game overall, especially when it comes to mulitplayer, but I have always enjoyed playing solo fights on Smash Bros, with a single enemy. These are a list of things that have made solo play difficult in this game.
AI shield response: The AI seem to block in response to a button being pressed, this causes them to block most attacks that you throw at them, and causes some attacks to be nearly impossible to hit with. The amount of guarding is like this at every level, but their proficiency increases as AI levels increase.
Level 1 AI: Level 1 AI are basically punching bags. I've had rounds where the opponent only attacked twice, and the only other actions they would do is guard, and follow me around. Sometimes they simply stand in place and let you beat them up; was this intentional for training purposes?
AI spam distance moves: AI will do this constantly starting at level 3. They will launch their ranged attacks 3-8 times before moving on to another move. Goku is the worst with this since he can also use the Kamehameha in addition to his other ranged attack.
AI follow you around like puppies: When an AI is not spamming its distance moves it will follow you around and the only way to shake it off is to knock it far away, after which the AI will continue to follow you from an obnoxious distance. This makes fighting strategic battles difficult. This also makes it very difficult to use characters with charge moves.
AI death falls: If you knock an AI off an edge and chase it, it will respond by air guarding and then falling to its death.
AI taunts: This one is not really a big deal, it can just be annoying. An AI will sometimes continuously taunt you when it is at a far distance from you. It will also do this every time you die.
As much as I enjoy this game, what the AI do can get quite aggravating when trying to play alone. I'm not sure if others have problems with this or if it has already been covered before, but I have not seen change in AI behavior between versions.
The only other problem I have really found with the game is that you are sometimes unable to move to any of the sides when coming off of the re-spawn platforms, which can cause instant death on Planet Namek.
Thank you for the creation of this cool game, I hope it continues to grow and improve.