I think the aliens and the alien spaceship at the end was a bit much for even an Indiana Jones film.
Yes, the other movies also have supernatural elements to them, but that was the limit: supernatural. While unreal and not of this world, they still felt grounded in some sort of reality, mostly magic curses or voodoo rituals. For most of the movie, Crystal Skull was going down a similar path, but the ending ruined the thrill and mystery by going too far. It's hard to accept Crystal Skull as a bona fide Indiana Jones movie when an actual alien spaceship takes off at the end.
And when compared to the 95% of Raiders, 85% of Temple of Doom, and 88% of Last Crusade, a 77% is very much a let-down.
Except the writer of Crystal Skull is writing again, which for the fans who didn't enjoy Skull is not a good sign.
And it got moved to 2021, so...