Are they?
A place like Somalia then would probably have lots of ethnic groups, lots of white people, lots of Asians, lots of different people. 85% of Somalia is made up of ethnic Somalians. The non-ethnic Somalians are in the south of the country. And yet their country is a mess.
The US has a Congress made up of 2 parties. That's it, you'd think therefore that like minded people would get lots and lots done, seeing as the Reps and Dems are kind of like minded in a lot of ways, and yet they get nothing done.
I don't see what you've said. Sometimes it is the case. Take Iraq however. Things got done in Iraq under Saddam, then he was deposed and the main religious and ethnic groups split and don't like each other and have caused problems. So, they could work together, except when people start causing problems between groups, mostly nationalists, and mostly nationalists are on the right of the spectrum.
Who in America is causing the religious issues? Mostly the right.
That's where a lot of the racists camp out, isn't it?
However racism existed in Germany in the 1930s. In 1900 Jews made up 1.04% of the population of Germany. For the most part Germans were Germanic. Yet they couldn't agree on shit, they had the Weimar Republic, Communists and Nazis started to do well, but there were so many political parties it was incredible with a whole host of views on how to deal with the world.
Come the modern era and the Germans have Turks coming in and not able to get German citizenship even after like 3 generations, but they let in "ethnic German" Russians who just drink and cause problems and don't even speak the language.
So again, what you're saying might have relevance in particular cases, but in other cases it might be meaningless. To blame ethnic groups in the US is a little strange to me.