Basically that it is too faithful to the original film and that it undermines/retcons the 2016 movie. Below is just one excerpt from The Guardian's review who gave it 1 star out of 5.
It’s pandering all the way down, the shocking part being the variety of Reitman’s ploys. It’s not all groaners like a cop offering a jailed-for-the-night Trevor the phone and asking, “Who you gonna call?” There’s the set piece with cutesy, nattering mini-Stay-Pufts scratching the itch for cloying mischief-makers planted by the Minions. Consider the casual cowardice of a script that uses its own mythology to subtly erase 2016’s all-gals reboot from the canon, giving the rage-choked trolls carpet-bombing IMDb with zero-star ratings the vindication they’ve always craved.