The whole 3rd gen line is highly under rated. I can't understand why.
I think we'd have to dissect the idea that 3rd Gens are under-rated. If you mean that now, here in the year 2016, they seem to not have a huge following and don't bring higher prices that might represent their ability and stamp on history, I might agree with you that seemingly -many- people don't realize how completely capable, durable and enjoyable these guns are.
However, if we look over their body of work since their late 80's introduction and the fact that these pistols found themselves in duty holsters all across this continent and even in many places around the globe... and we also look back at
how many they managed to produce
and sell (and this fact is hammered down with EVERY new group of re-imports or "former LE batch" of guns recently made available for sale...) it would simply be wrong to say that they were under rated back in their day.
Obviously, the bulk of the market wants a polymer frame and a striker fire design today. It's not like the Beretta 92 is killing the market that S&W has seemingly abandoned (it is not) and DA/SA (some call it TDA) guns are simply an older design that many or most would agree was fine for the time but that time has since passed.
I suppose I can't really see the 3rd Gen pistols as under-rated, but really, we kind of just end up discussing the definition of "under rated" as much or more as we do the pistols themselves. I would imagine that almost ALL of us can agree that simple economics has helped doom the 3rd Gen line. You only need to look at a completely bare, stripped, empty 3rd Gen frame to see every little hole, rail, edge, and spot where that steel had to be milled or cut or worked over and you can see why and how a for-profit manufacturer simply
HAD to move on from building them. They can not only sell ten M&P pistols in the time it would take to sell one 3rd Gen... they can probably make three M&P's for the same manufacturing cost as a 3rd Gen and they can do it infinitely faster with far less actual work, wear and tear on equipment and sadly... they can do it with a lot less human skill.