Agreed. Supposedly the M&P designers put it under the rear sight so that it would be farther away and less susceptible to fouling coming from the chamber.
That's what they told us in one of my M&P pistol armorer recert classes.
Bear in mind that S&W has had many years experience with 2 difference designs of plastic pistols, both of which have had the safety plungers located at the front of the striker/FP channel.
I don't see the problem with the safety plunger being located toward the rear (where it
is less exposed to fouling and debris from the chamber, via the breech face hole, during live-fire), myself, but as an armorer I'm not inclined to frequently remove the safety plunger, anyway.
On the other hand, now that the safety plunger is located close to the slide's opening cut at the rear of the striker channel, it's probably more susceptible to potential
owner/user contamination which could result from improper cleaning/lubrication practices. (Just like getting unwanted exposure to solvents, CLP's, cleaners or oils into the striker channel via improper and/or inattentive cleaning practices.)
One nice change the engineers have made is to revise the safety plunger's spring plate to have a nipple onto which the safety plunger spring can be affixed. Makes it a lot easier to install the plunger spring & spring plate ... without the spring plate living up to it's earlier name in the armorer classes, which was the "UFO".
Having 1 (or even 2) set screws isn't uncommon among rear sight bases, since there's no way to predict whether the normal tolerances of either a particular sight base or slide dovetail cut is going to be snug & tight enough to resist lateral drift under recoil forces. The set screw usually just snugs down on bare metal underneath the sight base.
In some 3rd gen models there was also an oval-shaped spring plate that covered the top of the ejector depressor plunger and the firing safety plunger, and the rear sight base's set screw would just snug down onto the spring plate.
