Hi Nadja,
I think it is very hard to make any meaningful observations when it comes to seed germination and viability.
Some growers have good results with the very same seed that others fail with, and at other times the success has been reversed!
All seed that is viable still requires the right conditions and we all, as growers, have different methods and conditions.
Some seed will germinate very quickly with a high % rate, others will take many months but will still give good % rates.
I suspect that we will never really answer this type of question as there are simply too many variables and too few opportunities to test any theory. (we cannot make the same seed frequently enough due to hit and miss timings of flowering, clone variation, etc)
The likes of the Dutch growers would have the best opportunity from a quantity perspective, however I suspect firstly that they do not grow any plants to flowering size, and what is more, they don't grow the cross specific to your question, and therefore, could still not offer any specific answers.
In my opinion, niche plants require niche conditions, and every Nep species is in a specific niche of it's own, so lumping them together simply is not possible.
We can fit many species to a range of condition or requirements, but they by no means all fit perfectly. This surely means the same for their respective seed and subsequent germination.
A plant that produces many thousands of seeds expects low % levels of survival, and even when sown locally to the mother plant, many simply fail to germinate due to imperfect landing sites. If we confine our sowing ultimately to one set of micro conditions, we can reasonably expect total failure!
A personal anecdote: I have put Miranda pollen to many different females, produced much good looking seed and germinated not a single solitary seed!
I assumed that Miranda was not viable, however others have produced crosses with Miranda.