You're creating a distinction here that does not exist in any shape or fashion, except for size. The difference between a state government and a federal government is simply over the scope of their decisions. In other words a federal government is no more guaranteed to make good decisions than a state government bad. The exact same sort of powers that enabled the US federal government to abolish slavery are the same sort that enabled governments time immemorial to enforce it. In contemporary times it's the same power that enabled the Nazis to enact their agenda on a national level. Many people try to convince themselves that such wrongs might never happen again on a national level, but is there any logic for this? To me it seems to be simply cognitive dissonance.

So the question is this: Is a government that is closer to the people that it governs, both figuratively and literally, more or less likely to create decisions that are beneficial to the people of that region than a strong centralized government great distances from them? There's a disconnect here that can bias what you see. Imagine a government falls to corruption and starts enacting negative policy at a rate of 10% per year. And now imagine we have 1000 sovereign governments. Each and every year you'll see some 100 different governments engaged in all sorts of awful behavior. Now let's swap it around and imagine we only have 1 single world government. In 100 years you'd only see 10 years of any bad acts. But the big difference is that when those acts were carried out they would be global in impact and their magnitude is something one can only imagine.