Basically, the biggest factor is cooling when running an i7 at 1.9v. What also comes into effect is how well the processor was manufactured. Not all cpu's are created equal and most have some sort of tiny, tiny flaw that won't ever surface unless the cpu is taxed/oc'd too much.
The more voltage you add, the sooner your cpu simply burns itself out. Also, and correct me if I'm wrong, the smaller the architecture, the less voltage you can safely run through the circuits. 1.5v was a good ceiling for running Sandy Bridge. 1.25-1.35v seems to be the general consensus for running an Ivy Bridge cpu.
Edit: Have you seen those videos of people overclocking the newest AMD cpu's? They're using liquid nitrogen. Achieving such a high speed requires insane cooling techniques.