I don't really think that matters. It could be either. It is also possible that there is no value judgement involved and a bare fact is stated as a fact. The question is whether the 'which' clause works here or not.
I've often seen this kind of construction, so it is in use.
I don't know whether it's grammatically correct but I suspect not, because "which" cannot properly refer to a conjugated verb, i.e. I can't see it here as a being a "sentential which". It can refer back to a gerund phrase:
They were in the habit of crossing the border illegally, which I have never done.
They crossed the border illegally, (which is) something I've never done.
They crossed the border illegally, a thing I have never done.