The German numbers are heavily skewed by East Germany, which is still below the living standards of West Germany. If you live in a wealthy, West German city (as I do), the quality of living is very high indeed.
The US is skewed by absorbing a vast wave of Latin American immigration in the last 30-40 years, which is a perfectly reasonable comparison to absorbing East Germany (which was 16 million in 1990).
I hear the argument but not sure it compares well. If one looks at Germanys consensus from 2013 they find that the east is different in every manner. Diversity, wealth, education. The difference here is that the effect from influx to the US from poorer demographics is they distribute better than what is seen in Germany which helps reduce the statistical impact.
US is absorbing immigrants from its south through a heavily defended wall, with an option to kick out of its borders anyone that it doesn't like (and this in fact happens). East Germany on the other hand, had to be taken in what is current Germany with everything it had, be it good or bad. Not comparable at all!