What problems are they causing?
Among other things, the uber-rich tend to buy politicians and bureaucrats who then make laws and regulations favoring the uber-rich, often at the expense of other people, including small business people and the owners and employees of mid-sized corporations. For instance, one tried and true tactic is for the uber-rich to purchase regulations that their own companies have the means to implement, but which cripple or impeded up and coming competitors. The practice reduces competition and drives up the cost of products and services to many people.
Put differently, have you ever wondered why during the last recession, the banks and financial institutions got more bail-out money than the entire rest of the US economy, but didn't start lending again to small businesses for quite some time?
Another thing the uber-rich tend to do these days is offshore jobs and capital. "Local millionaires", so to speak, tend to invest something of their wealth in their local communities. They also tend to invest something of their time and energy in local causes, organizations, and issues. So, for instance, you will see local millionaires funding their churches, mosques, temples, and synagogues, which in turn, often perform community services. Uber-rich don't as often make such commitments, but that's a very minor point compared to....
Uber-rich seem much more willing to invest in lucrative industries that use techniques and practices which destroy local environments, such as strip mining the Appalachians. And...
They seem much more likely than local millionaires to hire PR firms that put out false or misleading information about the benefits and/or harms of their practices.
But the main thing, in many ways, is their disproportionate influence on Federal, State, and local governments, and regulatory bodies. As Chris Hedges summed it up, no American today can cast a meaningful vote against Goldman Sachs in a presidential race because any candidate with a real chance of winning on either side is beholden to Goldman Sachs.