NSL for those kinds of roads are mainly just less hassle and maintenance I believe. You might have stretches where 60 is all good, then somewhere even 30 is too much. Why maintain two or three extra signs (or pairs of signs) when you can just have the NSL signs cover the whole thing. Okay, so why not just blanket the road with a lower limit? Same again really. Is it worth the hassle and cost for a quiet road? Probably not.
To add to that, these roads aren't usually as high traffic, just making the change less justifiable. It'd make sense on some more frequently used country roads. For example, I have a few near me which are pretty solid roads, well used and NSL+ can be done quite easily on some stretches. But when we're talking single track kinda stuff? It's hard to justify spending the money on new signs and maintaining them. Even less so if people are 'policing' themselves because those speeds just aren't safe on that road.
Simply put, this is basically one of those scenarios where the limit is just that. An absolute limit.
As for what is too fast/slow. It's a tough one and depends on a road-by-road basis, that's before you throw in different scenarios. All I can say to that is use your best judgement. As long as you're in full control of the vehicle and within the speed limit, it'd be hard to get slapped for going too fast.