I'd certainly agree those two films were better than this, but it was always going to be difficult for it to be as good when introducing so many new characters. (And I don't buy for a minute that The Flash, Aquaman or Cyborg would command enough screen presence to justify their own films.) I think it would be fairer to judge the next installment more critically personally. This felt like a warm-up to potentially better things - although I fear it's always going to struggle when led by Cavill and Affleck unfortunately.
Put it this way, for Justice League to have 40 on Metascore, yet Age of Ultron have 66, suggests an inherent bias to me. Because that was a very monotonous superhero film as well and far more underwhelming given the humongous backstories it had in its favour.
Where was Lex Luthor by the way?
Age of Ultron was a better film. It had characters people were invested in, a team that we'd only just seen come together in the previous film that was now a proper unit, plenty of fight scenes where they work together in interesting ways and a better protagonist, albeit a CG one. It wasn't an amazing film and on repeat viewings the quip count becomes a bit annoying, but I easily enjoyed it more than Justice League.
I'm sure it was difficult to fit all of these characters together in one movie so soon, but no one made them do it. They could've at least made a Batman movie, a better Superman, maybe introduced some of the other characters in these solo films first, or given the team a less deadly threat to deal with first, but instead they went as big as they could as fast as they could and it resulted in a bad movie. Hell they could've made an entire trilogy of Batman movies before getting to the old, gruff Bats of BvS/JL and it would've worked well.
Also it's easy to say that the other characters couldn't carry their own solo films, but then the same probably could've been said of Thor or Iron Man at one point.
I'm assuming Lex is still on the editing room floor somewhere.