I've been told that S&W periodically buys representative samplings of the major American ammo company products to shoot in their guns. Last I heard, each M&P is fired for function with 15 rounds of factory ammo, using 3 test mags (not the ones boxed with the guns) loaded with 5 rounds each.
If an owner uses a less than satisfactory grip technique (including "breaking" their wrist and not locking it behind the shooting hand), or uses budget ammo that might run a lot lower in power when it comes to normal power levels, it's not surprising they may not experience consistent ejection patterns or weakened ejection.
Glock's Gen4 guns were pretty much initially sprung around the .40, and they realized they finally needed to use different recoil springs for the lighter recoiling 9's. They also ended up revising their ejectors in the Gen4 guns (and, FWIW, in the Gen4 .357 guns they further revised the ejector
tip angle compared to the previous generation of .357 ejectors).
S&W engineers have been doing with their M&P pistols what they did with their 3rd gen guns (and are still doing in occasional LE runs of 3rd gen's), which is to constantly look for ways to revise and make subtle improvements in design and manufacturing. They've changed quite a bit since their release in '06.
Think of the factors influencing the functioning of the gun as being ... the
gun (of course); the
shooter (grip technique & experience); the
ammo selected (power, QC, etc); the
maintenance practices used; and the
environmental conditions in which everything is actually used. Once the gun leaves the factory, they have no control over the shooter, ammo, maintenance practices or the conditions in which the gun is used.
Now, picture all these influenced as being a large Venn Diagram, and start imagining all the myriad ways these disparate influences can be shifted to overlap and be present in varying degrees.
Sometimes you get what you pay for, and this includes using really low cost ammo.
