Yeah, I mean, Cali is a huge mixing pot. But, yes sadly they suffer from some of the issues that Mexico has. And yeah, it does affect our violence rates. But there's also simply a lot of ethnic divide there. There are certainly things to recommend Cali, but it's not really my cup of tea personally.
Don't visit the South, unless you go to larger areas, or are going to Florida for the beaches, because there's a lot of racism in those parts of the country. The south can be nice, I just don't recommend it if you aren't used to America. The upper East Coast is definitely a great place to visit though, there's a lot to see and do and is general more open-minded in my experience.
Yeah, I mean the thing about the US (and I've explained this to people here a few times and they kind of don't understand) is that there's a lot of emptiness in the middle. The large majority of our country is on the East and West coast, with not much in the Midwest areas. But you're right, there's certainly different cultures separating most of the states, but the one thing you can give Americans credit for is their love of their country. As an American, you do find that you can be very different from other people in the country, but largely they will still accept you. I suppose that's an upside, but there are areas that have animosity towards one another (the north and the south in particular).
Anyway, I hope any travels that you have here are both fun and safe. I hope to visit Spain some day, myself. Europe fascinates me because, like you said, there's so many different kinds of cultures really closely packed together.