The inclusion of so many characters from so many movies is impressive as hell as are the VFX in both films. It’s difficult to imagine Marvel, or any other studio, making a more epic crossover. That said, I have two main criticisms.
The Infinity Gauntlet was one of my favorite crossovers when I was a kid. It was basically meant to be Marvel’s version of The Crisis on Infinite Earths. It ended up not being as good, but did sell very well and used the same artist from Crisis, George Perez. The movies were a nice nod to the story but didn’t follow it. Thanos’ motivation, for instance, in Infinity War makes zero sense. Apologists have used the excuse “Well he IS crazy,” but that’s not good enough. He couldn’t be stupid enough to think that killing only half of all life would cure overpopulation for more than a couple of decades. Since I have been alive the world human population has nearly doubled. So in 50 years all of his work will have been undone. If Thanos did painlessly kill every sentient in the universe then he would ironically become the greatest hero of all time as he would have ended all current suffering and all future suffering.
In the books he chose half because it would be an impressive amount of beings to kill without really devastating the order of things. He did it to impress an unimpressable woman, the female personification of Death. It would have been much more interesting to go that way.
The deaths in Endgame were so obviously used as a way to get rid of expensive cast members, so the three heroes who died were unsurprising and expected. But the worst idea they had was how to handle Cap’s exit.
Cap is another character who I have been following since childhood and one thing that he is not is a goddamn quitter. It’s the same problem I had with Nolan’s third Batman film: he not only quits being Batman once, but TWICE in the same movie! Bruce Wayne and Steve Rogers are very similar characters in many ways. They are both idealists and neither one of them ever married. There is a reason for that: RESPONSIBILITY. As in, “With great power…” you know, the most famous line Stan Lee ever wrote.
For some reason the writers of the movie and the directors cannot even agree on what happened. Either Cap created an alternate timeline where he could settle down and none of this ever happened or he went back in the current timeline and just sat around watching TV while Bucky was being tortured in Russia. So that would mean that he just let all this happen again and did nothing. That is just not acceptable if we know Cap to be a courageous, self-sacrificing superhero. Key word “super.” He’s not a normal soldier. He doesn’t relax while evil exists. He doesn’t retire. He doesn’t quit to grow old in peace and quiet. Because there can be no peace until evil is wiped out and that’s a war that’s never-ending.