Execution is extremely important especially when the script for your film is simplistic nonsense that cancels itself out by the end, rendering the entire proceeding plot null and void. Audiences respond to excellent scripts and acting. They respond to beautiful looking imagery and well-crafted visual effects that make them believe the impossible is real. The suspension of disbelief relies on the audience forgetting that they are watching a movie. When you cram in a bunch of obvious remember-berry cameos into your bad CGI fest, you end up pulling the audience out of the fiction.
No Way Home and Endgame worked so well because of the fact that the audience was deeply invested in the characters and plots. The same cannot be said for the Flash film. This film didn't build a franchise that led to this moment. Warner's problem has always been that they are so desperate to emulate Marvel's success, they skip important establishing steps and rush to the Infinity War-like finales without earning it. They rushed their Batman V Superman, they rushed Justice League, and now they are pushing a Flash film that should be the third in a series of films, not the first.