Eight-ball pool and nine-ball pool are two completely different games, and both difficult in their own ways.
With 8 ball, you need to be smart, and know when to go for the run out, and when to play safe. Also, with 8 ball, you will often have to deal with clusters, so those clusters need to be broken out, before you can win the game.
Nine ball is easier, in a way, because with 9 ball, you often do not need to deal with any cluster situations, and the hardest part about 9 ball, is getting good shape on your next numbered ball.
So, they are just different, and difficult in their own ways. I was always a much stronger 9 ball player, and a pretty weak 8-ball player, in comparison. I disagree that 8 ball is a beginners game. It is a game that all beginners start playing before any other game, but it is a great game, and takes smart thinking, in a lot of situations that you may find yourself in, while playing 8 ball.