What are Charlemagne's successes and failures?

Charlemagne (r. 768-814), also known as Carolus Magnus or Charles the Great, expanded the Frankish Kingdom into the northern Balkans, Hungary, Bohemia, Italy, eastern Germany, and northeastern Spain. His court promoted meritocracy by employing the most skilled workmen and brightest minds from across his kingdom. Charlemagne's coronation as emperor, or augustus, by Pope Leo III (r. 795-816) set him up as a rival to the Roman emperors in Constantinople and thus signaled western ambitions for independence from the Byzantine East. He worked toward establishing a centralized state with a comprehensive legal system, a network of churches and schools, and rudimentary infrastructure.
Charlemagne had some failures and moral flaws as well. In 782, he had thousands of Saxons murdered for having lapsed in their Christianity. He failed in his quest to marry the Byzantine empress Irene, who reigned in Byzantium from 797 to 802. Finally, Charlemagne's empire fell apart soon after his death. Some historians blame his successors, but one wonders how sound the empire was if it fell apart almost immediately after his death.
Einhard's "Life of Charlemagne," linked below, provides more direct information on Charlemagne's life than any other source; however, it should be read with a critical eye and in tandem with other sources, since Einhard considered the king a benefactor and father figure.
https://sourcebooks.fordham.edu/basis/einhard.asp

Charlemagne was one of the most lauded kings of the medieval era. During his reign (768-814), he led more than 50 military campaigns and conquered nearly all of western Europe, metaphorically rebuilding the Western Roman Empire. His empire, in fact, became known as the Holy Roman Empire and created a close link between the Church and the state. He was such an efficient and organized military commander that his campaigns inspired Napoleon.
Unfortunately, Charlemagne's success as a ruler did not extend to his descendants. Charlemagne's son, Louis, was a weak ruler; his sons, in turn, divided the empire into three parts at the Partition of Verdun (843), and the empire further dissolved from there. Thus, one could say that while Charlemagne successfully built the Holy Roman Empire, he failed to ensure that the empire would remain strong after his death.
https://www.spanport.ucla.edu/santiago/histchrl.html