Main reasons why the CGI "de-aging" looks weak
With the film's massive budget (159 million) and nearly two years of post-production work you would think the CGI would be outstanding. However, there are two big reasons why it does not work.
1. De Niro was very stubborn and uncooperative by refusing to where face cameras and motion capture dots on his face saying that he would never do it and the visual effects artists had to figure out other ways. Well, it would have been a lot more realistic and believable if he did it the former way. Instead it looks creepy and not effective, when De Niro was called "kid" by Pesci, I actually laughed a little. Here is the link for proof https://www.latimes.com/entertainment-arts/movies/story/2020-01-02/irishman-de-aging-of-hollywood-how-they-did-it
2. They should have had body doubles for one major scene. The beat down that De Niro gives the grocery store owner was really pathetic. It looks like an elderly man giving a half-assed pushing, punching, and light kicking to a over exaggerated grocery manager. Did Scorsese or for that matter look at the dailies and say "yeah, this looks amazing" or was everyone to embarrassed to tell De Niro it looks terrible and should be re-shot with a body double, because they certainly had the time and money to graft or integrate De Niro's face onto a double's body.