I don't think there is a browser that uses a full thread-per-tab model, so there's really nothing to compare it against. The problem with other browsers is that slow JS or in some cases flash in one tab will slow down other tabs.

Also, the tabs in a browser running many sites are much closer to traditional use of processes. Web application servers running the same codebase on each request seem like a better fit for threads. For one thing, the security model for the two uses of JS are very different.

As for robustness, if doing X will cause a crash and your code does X, then you will just have a bunch of crashing processes rather than just one. How is that better? Wouldn't the real solution be to either stop doing X or fix X so that it doesn't cause a crash?