Complex sentence: Not one of his librettists managed to produce a libretto on the same artistic level as Rameau's music (the plots were often overly complex or unconvincing), but this was standard for the genre and is probably part of its charm.
Split into simpler sentences: Not one of his librettists managed to produce a libretto on the same artistic level as Rameau's music: the plots were often overly complex or unconvincing. But this was standard for the genre, and is probably part of its charm.