Hello Avatar, I think it rather has to do with the Japanese culture. While in some parts of the world you would be be encouraged to show your individualist side, take initiative and tell people about your accomplishments, in Japan you need to be humble. If you compliment anyone ,they take it as a formality and will probably deflect your compliment and come up with some kind of excuse, that makes them look worse. So maybe he rather means: "Complimenting Japanese people is very difficult." They would not accept it or be pleased if complimented and they will rather try to turn it aside.

The speaker seems to have difficulty saying nice things to people (complimenting them) in Japanese. He/She can't seem to find the correct words and phrases. What is not said here is whether the problem has to do with the speaker's poor grasp of Japanese, or with the Japanese language itself.

用日语恭维人很难。 this sentence kind of make no sence.