Perhaps Merkel was simply describing German reality as she sees it. If one is honest, rather than ideological, it is hard to think of a European country in which "multiculturalism" has been a great success. In a sense it has been a success in Israel, but it was multicultural to begin with, with immigrants from all over the world. In the late 19th century mainly from Russia and other parts of eastern Europe, but soon followed by people from western Europe, the middle east and north Africa. There was, of course, the common element of being Jewish, but with vast cultural differences between, say, those from Austria and those from Morocco or Yemen. And even today, one can notice us-and-them attitudes between European and African or Asian Jews. I even once heard a taxi driver, of Iranian descent, railing against Iraqi Jews because he had married one and she was giving him hell.
Multiculturalism is a wonderful ideal, and I'm sure Merkel would have liked to see it work smoothly, but better to describe things as they stand, implying a need to do better, rather than uttering platitudes.
Incidentally, in the Guardian article the reported stated, actually stated as a fact, that in saying what she did Merkel was "courting anti-immigrant feelings." And that is editorialising, rather than reporting.