Welcome Archangel, hope to see many thoughtful posts from you.
Puzo's pre-GF novels, "The Dark Arena" and "The Fortunate Pilgrim," got good critical reviews and were considered "artistic" works--that is, "serious" novels according to critics' standards. But they went nowhere commercially. Puzo wrote "The Godfather" in part because he needed money to pay off gambling debts. So, instead of following the "artistic" formula of the first two novels, he wrote a "popular" or "sensational" novel to make money. Was he overly critical of "The Godfather"? He's certainly entitled to his opinion of his own work. But, I'm one of his readers. I read both the early novels and didn't like them. I've read "The Godfather" hundreds of times, and loved it each time.