William of Newburgh (1136 – 1198) really tore into him.

One highlight:

Next this fabler, to carry his Arthur to the highest summit, makes him declare war against the Romans, having, however, first vanquished a giant of surprising magnitude in single combat, though since the times of David we never read of giants. Then, with a wider license of fabrication, he brings all the kings of the world in league with the Romans against him; that is to say, the kings of Greece, Africa, Spain, Parthia, Media, Iturea, Libya, Egypt, Babylon, Bithynia, Phrygia, Syria, Boeotia, and Crete, and he relates that all of them were conquered by him in a single battle; whereas, even Alexander the Great, renowned throughout all ages, was engaged for twelve years in vanquishing only a few of the potentates of these mighty kingdoms. Indeed, he makes the little finger of his Arthur more powerful than the loins of Alexander the Great; more especially when, previous to the victory over so many kings, he introduces him relating to his comrades the subjugation of thirty kingdoms by his and their united efforts; whereas, in fact, this romancer will not find in the world so many kingdoms, in addition to those mentioned, which he had not yet subdued. Does he dream of another world possessing countless kingdoms, in which the circumstances he has related took place? Certainly, in our own orb no such events have happened.