I've expanded a little bit more in another comment here, but I think 2) is less a case of any fundamentally different level of isolation, but more a maturity of

I've expanded a little bit more in another comment here, but I think 2) is less a case of any fundamentally different level of isolation, but more a maturity of design and implementation. While Zones were designed as first class, containers were not, they're built out of a variety of other building blocks, which while they may be more powerful primitives, have seen less review of design and implementation and exposed a large attack space on the kernel. User namespaces are the prime example, possibly because they are the largest/juiciest new target, but I expect we'll see more with time.