That’s a gross oversimplification. The criticism was towards the third act and Jean dying according to actual reports from Deadline:
https://deadline.com/2019/06/dark-ph...hy-1202629749/We heard that in one cut, Jean Grey dies, which wasn’t received well. But overall, the major ending change-up, executed in reshoots, entailed going from an intimate ending with Jean Grey (Sophie Turner), Tye Sheridan (Cyclops), and Charles Xavier (James McAvoy) against Jessica Chastain’s Vuk. The feeling from the audience in testing was that they wanted to see all the X-Men heroes fighting in the end.
Nothing about characters not getting fleshed out.
Eh, I guess we will just have to wait and see.