One of the things S&W tried to avoid with the M&P, in my estimate, is a very short and light trigger. I think they could have engineered a lighter trigger in there, but decided not to, and not just for cost reasons. Why? Because it would make the pistol easier to trigger, literally, and thus dangerous in "everyday" use as a SD or service pistol (and these are pistols that by and large do not have safeties).
Would you leave your S&W revolver cocked, just so it had a very smooth 2-3 lbs. trigger? No, you leave the hammer down and have a very smooth but long and heavy - 8-12 lbs - trigger pull. Because it is safer. So safe, in fact, revolvers don't have "safeties." The long trigger pull IS the safety. Likewise, the longer and heavier trigger pull on a M&P is safer and more appropriate for what most people use the pistol for.
Apex triggers are fine but they are generally for people who practice with their gun and know it well. They are super on the range. But if you have an Apex kit in a self-defense gun, it might be wise to use the heavier of the two springs provided by Apex in the forward set sear trigger, and it certainly is smart to think about what you've got there, and to be aware what you are working with. Among other things, it is a pistol that is a bit easier to trigger and thus to have a ND when holstering, and in fact in while handling it in general. I know, I know, the best safety feature is between the ears. Which is why NDs keep happening.
I'm just putting forth this line of thinking as an explanation why stock M&Ps have a somewhat heavier and longer trigger, from what I imagine is the perspective of the M&P engineers. (The grittiness IS a product of cost-savings, but works itself out after a couple of hundred rounds or, as indicated, with a light stoning).
My point it, we shouldn't lament the somewhat long and somewhat heavy trigger of the stock M&P -- it is appropriate and reasonable! For those who want a lighter and shorter trigger, there are Apex kits.
My M&P9 has a nice, light Apex trigger & FSS kit, and is very smooth. Not quite the smooth and light trigger of cocked S&W revolver or a good pistol when in SA mode, but pretty darn nice. My M&P is NOT my home defense or carry weapon -- it is a range gun. My HD weapon is a 3" Mdl 65 and my CCW is a Mdl 642.