I also think ChNN gives very solid justifications for his approach. It makes perfect sense. I'm sure you are quite aware of them, Magnus.
It's not that Ngöndro isn't useful. It may be very useful as any other well done practice. It's just not the main point by itself. I would say it can become a hindrance if one gets so attached to it that fails to realize its own essence. From a Dzogchen perspective, it makes much more sense not to be obliged to perform any particular practice with fixed numbers, goals and so on. There are very useful practices one can do to overcome obstacles, whatever they may be, wherever they come from in terms of yanas. However Guru Yoga is the main practice and this makes perfect sense from a Dzogchen perspective. What doesn't seem to make sense from a Dzogchen perspective is the idea of set of templates that fit all practitioners, whatever their circumstances and capacities, to do a full circle and end exactly where they started. Is Ngöndro useful? It can be very useful and I speak from experience. Indispensable? Not a chance. It even goes against the Dzogchen view to say such a thing and although excellent masters recommend Ngöndro, none of them is more of an authority than ChNN to his students.
There are many ways to climb the mountain. So far, ChNN presentation seems the fastest of them all to me. Unfortunately, its so subtle that many miss it, even while being his students. It's so easy to screw up that we must be very careful so that we don't understand ChNN wrong, and this is easier than it seems. In my own experience, I really felt I should empty the cup when I became his student, otherwise I would be mixing my own preconceptions with his teachings even without noticing (I'm sure I still do, but to a lesser degree). The result of such mixture may vary. If having a solid background is definitively helpful, we must keep the pliancy to learn something new, sometimes scandalously new. If we get him, that shows in practice very fast. This has been my experience after spending more than a decade with the tantric approach and a lot of time available to practice in quite good conditions. You just can't compare it. This has been my experience and so it is valid for me and me alone.
I realize ChNN teachings may not fit all people, so everyone should do as they think it's better for themselves. I don't even recommend ChNN to a lot of people these days as I find his teaching so subtle that I guess the chances of not getting him are very high. When I started, his teachings seemed easy, very accessible and nothing special in terms of deepness. That was because I wasn't getting him at all. I think it it doesn't even matter how much one studies. There's a point when it clicks and the pieces come together in such a harmony that you have one of those memorable "aha! moments" that will stay with you from then on. However, if the secondary circumstances are present, people will get to practice Dzogchen like that, so there's really no need to say or recommend anything. I'm quite confident that nobody meets ChNN by chance. Then it's up to us.