It's not that far off in terms of team size, 7 or 8 by Guardians 2 vs. I think 9-10 for Eternals.perhaps a counter to that is Guardians of the Galaxy. Which made a bunch of Marvel Nobodies work well.
A key difference is the smaller cast. How many eternals were there?
Probably another thing that bugs me with Marvel is how many completely different secret species they keep tucking away on earth. Eternals. Inhumans. Mutants. what else. Stick to one concept so we don't have to get into continuous "and why didn't THEY help with Thanos?" Marvel does a good job with individual characters and making them fit when they bring them in.
What works for Guardians is that all the characters have goals and interests. Things they are sensitive to, ways of solving problems, etc. and a lot of the first movie is spent discovering those things on screen.
This is vs Eternals, where the approach was "these characters have history and personality...off screen but we're just going to rely on little bits of actor charisma to convey that during the movie instead of sharing that history and personality with the audience". And that's not even considering that half the team was functionally absent for almost half the movie.
It sort of makes you feel like the movie is a season finale for a show you didn't watch.