LVL 9 CPU have frame-perfect shields, rolls, air-dodges, and punishes. They are programmed specifically to read your inputs and react to them instantly, if their character allows it; their reaction time allows them to pull off things that no human would be able to perform consistently. They have built-in strategies and programming, so how they play may not be a good representation of how actual users of that character play, and as such you won't
really learn about that character's capabilities and what to watch out for. They all have habits; if you find an AI exploit that works against one character, chances are it will work for another, and no matter how many times you punish their habits, they will never stop or adapt to what you do, they will continuously do what they were programmed to, no matter how much time passes; unlike people, these CPUs do not learn. Perhaps it's different for Amiibos but I'd be careful with those as well.
They can be good for practicing basics, but typically, beating them involves exploiting their programming, and this isn't true just for lvl 9s, this is pretty much the case for every CPU level. They are not good representations of actual people; they will not help you learn in-depth strategies; things that work on them will not work on many people, and things that work on people will not work on them. If you play them too much, you may potentially develop bad habits that will be difficult to get rid of, and then you will kind of play like a computer yourself. Offline play is your best bet at practicing.
I'm not saying never play CPUs ever, because sometimes you may just want a quick, no-hassle match, or just some stress relief, what I am saying is when you play them, be mindful of the fact that they're not real people, and don't let them shape your playstyle too much. Hope that explains it
