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Of course people who are watching the movie are a fan of the genre and therefore have a bias, this is why the ratings for marvel movies are always extremely skewed: their crazy fans vote any movie 10/10 regarding of the quality.
''Almost literally a case of 'if you've seen one, you've seen them all'.''
Very true sir.
Regarding the CGI in the movies: yes, they feature one of, if not the best quality CGI. However, good CGI is not achieved by spending a shit ton of money (what Marvel does), rather, being creative, doing things not done before, having a good feel of the ambiance of the scene you want to achieve and combined with the choice of CGI and screenplay, all of this processed with a technical high level of CGI creates a visually stunning movie.
Just spending money on highly detailed visually impressing CGI with no regards to creative directions and scene ambiances creates a horrible mise en scene.
The CGI of marvel movies are high quality, yes.
The settings, decor, lightning, depth of space, costumes, make-up, emphases, color, balance and other things are all very mediocre since all marvel movies have to be the same: that's the shit that sells. This creates a rather poor viewing experience.