The tanks are soft plastic which screws into hard metal (or the metal screws into the plastic, depending on regular size or mini). Eventually the threads in the plastic will erode to the point where they call it a life, but if you're careful to not cross-thread, that life can be very long indeed.
You can also lose or mash the white plastic o-rings and thus cause leaking. I can't find replacement o-rings, but I've been successful in stopping leaks at the base by wrapping with plumber's teflon tape.
Finally, some
e-liquid flavors don't play nice with the plastic and can crack and/or dissolve it. You can get metal replacements for the plastic cylinders, but sometimes these don't mate well with the white plastic o-rings. In such cases, teflon tape again comes to the rescue.
In conclusion, the VVN tanks aren't bullet-proof. But they're close.