An undergraduate degree in physics in Germany will usually leave you better prepared for PhD work than an undergraduate degree in the U.S. Of course there are plenty of people who are individual exceptions, but what I'm saying is that the two degrees are not perfectly aligned. You need to keep in mind that the German equivalent of the U.S. high school is not perfectly aligned with the U.S. high school "diploma" either. You would have to do U.S. high school plus one or two years of U.S. college to be at a comparable level to the entering first-year college physics student in Germany.
I am basing this answer on what I have heard from my spouse who studied physics in Germany, and my spouse's comments over the years about what our son did in high school and college (physics major) in the U.S. where we live.
But the best way to check if my answer still holds would be to compare programs of study, course descriptions, and syllabi.
Bottom line, for physics, at least, you can probably go comfortably from a German undergraduate degree to a U.S. PhD program.