Leaving aside the uneven pacing and the iffy humor, this movie didn't work for me for two reasons:
1) I am perfectly happy to suspend disbelief for a movie, but the movie needs to be internally consistent. If a two hundred pound man shrinks to the size of an any by having "his molecules get closer together", why doesn't he still weigh 200 pounds. If he is light enough to ride a flying ant, how is he heavy enough to throw around full sized people. And, if he really is that light, why doesn't Newton's law make him fly through the air when he hits people? All through the movie, the logic made him light as a feather when the story needed him to by light, and weighing 200 pounds when they needed that for the plot.
2) The instant hero problem. He had three days of training from the first time he used the suit, and he was already enough of an expert to flip through the air, fight a bunch of guys and control hundred of ants all doing specific and different actions. Corey Stoll had less than three days of practice, and he could do all the acrobatics, and fly as well. Remember the great training scenes in the first Iron Man. That was good writing showing a realistic use of unrealistic technology. This movie had none of that. That fact that Any Man could win a fight against the Falcon after two days of training is an insult to the Falcon and to the audience's intelligence.
Then there are all the smaller issues. If Any Man has gone small enough to slip through the molecules of Yellowjacket's backpack, why is he still big enough to break through wires and pull out chips from inside the backpack? How did Hank Pym have any idea what happened when you went to the subatomic size? Why did putting an enlarging disc in the regulator make him grow again. That was a totally difference functioning of both devices.
As I said in my title, I think this movie was not the worst Marvel movie ever. That honor probably belongs to one of the Hulk movies or one of the original Fantastic Four movies. But it is certainly, IMO, the worst one in a very long time.