In the real world, the Armour would crush flat when it his something solid(Like a tank shell, for instance...).
But this is movie magic, so these things are incredibly strong. Strong enough to be able to resist ANYTHING, with only flaking paint and a couple of bashed panels(But not in any way to hurt Tony inside).
I can accept the invulnerability of the armours. It's all part of the suspension of disbelief.
With that in mind of course, the fewer parts the armour is made from, the stronger it would be(But the more specialised the system of putting the armour on and taking it off, RE the Gantry). Another thing to do with the strength is that we have seen in this movie and the previous ones that the suits usually have more than one way to put them on or take them off. This would require drastic over-engineering. But i can buy it. Tony is meant to be a genius(And i expect Jarvis has a lot to do with the manufacturing of the suits too). Like, in Avengers Assemble, we see that the MKVII is "transformed" into what amounts to a missile, and can assemble itself onto Tony in mid-air(And in that scene it encloses itself around him from the back). But then in Iron Man Three, we see the MKVII standing on a kerbside and when Tony walks up to it, the *rear* opens up like flower petals to accept him walking into it(One of the coolest ideas in the movie, by the way: The MKVII is parked at the side of the road, next to cars!). I can't imagine an engineer in the real world being able to make the same suit be able to do that, but this is the movies, and i love the idea that the armour is that versatile(I have zero problems imagining that, in addition to those two methods of getting into the MKVII, that Tony could walk out to the gantry walkway on Avengers Tower and the gantry could assemble the MKVII onto him in pieces too, his engineering is that versatile).
These are all just facets of what makes Tony Stark such a genius in the movies.
But, *movie magic time*, i don't think the newer armours are any less strong than the older armours. I just think it points to the fact that the Extremis-powered-soldiers were so badass that they could tear through an Iron Man suit like paper. I can't believe that tank shells, or the Iron Monger, or Vanko's drones, or the Chitauri would damage those suits.
But the Extremis guys were the match of them.
I fully expected the after credits scene to be Tony being injected with Extremis. Or at the very least, working with the formula on his computer and announcing that he had made it stable. I can only expect the next evolution of destroying the armours(And losing his Arc Reactor), is another high energy power source like Extremis.