How to evaluate Bai Juyi's Song of Eternal Sorrow?

Bai Juyi's "Song of Eternal Sorrow" is well written, and the sentences are very rhyming and beautiful. And wrote a love story between Tang Xuanzong and Yang Guifei. And then it was tragic. His poem had a great influence on later people, and then let them take history as a mirror and criticize the society and emperors at that time, so it can also let us know the historical facts at that time.