No one likes to hear this, but it's usually poor shooting form.
People that have been shooting for a long time, develop habits. Some are good and some are bad. Add a significantly different trigger and those habits are amplified. If it's a better trigger, most will shoot better. If it's a poor trigger, most will shoot worse.
In the case of the M&P, the trigger is not great. But remember, you bought a cheap gun. It's unreasonable to expect a great trigger. I'm not minimizing the cost of the M&P. $350-$550 is a fair amount of money. However, in the world of handguns, these are at the low end of the price spectrum. To get this price, they mass produce. This means that tolerances have to be a little more generous so they will all fit and be reliable.
The biggest problem with the M&P trigger is the over-travel. It has a lot of over-travel and this can pull the shot off line. There are several ways to fix this and most are inexpensive, but require some skill with fabrication and mechanical things. By far the easiest "fix" is to install the Apex Forward Set Sear(FSS) kit. Of course then people complain that they shouldn't have to spend money...blah blah blah.
All of this can be fixed for $0 if the shooter works on their form.
- If the trigger is pressed straight back, rather than at an angle.
- If focus is on the front sight.
- If the trigger finger is the only one that moves.
- If the grip is firm, but not "gorilla" tight.
- If the shooter doesn't press with the heel of the hand.
- If...
- If...
Then the claim is made, "But I've been shooting for 30 decades and this is the ONLY pistol I've ever had trouble with." Usually this comment is made by those that have been happy with groupings that aren't really all that small, but surround the approximate center of the target. Heck, I even know one guy who shot better groups with his M&P, but because they weren't in the exact center, proclaimed the M&P to be a horrible gun.
Some of these claims are legitimate problems with the gun. S&W has fixed every one that I know of. Most of these issues have turned out to be the shooter though. Look through the posts here and you will find hundreds (I'm not exaggerating) of posts where the Point of Impact (POI) was corrected by improved form rather than adjusting the sights or changing the trigger.
I believe people when they say they've been shooting for years. I believe them when they say the M&P is causing them trouble. Even so, we tend to want a mechanical fix to a software problem. Sorry, if that were possible we'd all have Olympic gold medals.
What I find most amazing is that folks don't shoot from a rest and then claim the gun is inaccurate. Then, when a rest is suggested, they refuse to shoot from a rest. It's like they know the error is being generated by the shooter and are afraid to admit it. Why is it such a horrible thing to shoot the gun from a rest? That is the only way to be sure it's the gun and not the shooter.