It depends on the circumstances but most the time it isn’t good. Generally most people know what their nationality is and what country they identify with. Playing for a different country whose language you don’t even speak because one of your grandparents was from there isn’t right and is depriving someone who is really from that country their spot in the squad. Eg: Matty Cash. Players like Musiala are different though because he has very strong ties to both England and Germany having lived in both for years whilst growing up and being fluent in both Languages so it makes sense that he can pick either England or Germany.