Aaron is very variable in difficulty because each round he gets a random set of cards to flip over. It is possible he will get triplets of several colors very easily and you will lose immediately or it is possible he will have a very low score and you will beat him easily. Most likely, I have seen him lean more towards difficult but not impossible to beat. There are not variable levels of difficulty.
Aaron is not particularly hard to beat. It isn't interactive at all - it is really just a variable scoring goal that increases every round. I've been playing with it for my solo games, but you don't really need to use it and I may stop since it seems like extra effort for no gain.
For what it's worth, I'm 4-5 against Aaron. What I like about it as an AI is that it is not fiddly in that it does not require you play Aaron's and your hands. You deal his cards out at the beginning of each round and have a score you have to match or beat to continue--sometimes it is more aggressive than others. It plays quick (15-20 mins) and is easy to take with you.