This is one of the stranger aspects of this. It's precisely in 20 and 30 mph limits where there is most likely to be (vaguely) adequate accommodation for pedestrians at least, on a separate part of the carriageway (i.e. the pavement). On your average housing estate, the roads all have pavements, yet I'd be the first to agree that roads like that should be the target of a more widespread 20 limit. Conversely, there are miles and miles of rural roads, mostly but by no means exclusively NSL country lanes, where there is genuinely nowhere for pedestrians to walk separately, and the limit is much higher. Take a fairly randomly selected bit of the A470: if there's a pedestrian just round that corner, there's nowhere for them to go, you can't pass them safely without crossing the centre line, and you can't cross the centre line safely because you can't see oncoming traffic. I haven't gone far enough along there on GSV to be absolutely sure of the limit, but I'd imagine it's NSL. Even if it's 50 or 40, that's still a shedload more than 20.
It feels like there's a bit of a parallel here on how the scheme has been "sold in" with LTNs. The messaging with LTNs has exaggerated the safety aspect, both with respect to the current risk and the degree to which it will reduce meaningfully for any given user after implementation. Because most people most of the time don't have a particular problem with safety, they see the measure as pointless or actively objectionable (because they have to drive further or more slowly). Their attitude might have been rather different if the schemes had been presented in terms of reducing noise and pollution, and making their residential areas nicer places to be outdoors in, other than in a vehicle.
For me, the principal benefits of 20 limits are similar. Sure, they improve safety, one would hope, but that's not all. They can make town centres just nicer places to be for people who aren't in cars, and we want to encourage people not to be in cars in town centres. Conversely, if there isn't much to be gained from reducing the limit on the outskirts of a town, where there's a pavement for residents to use and no-one else much goes there on foot, and the houses are sparse and set back from the road so that any reduction in noise is negligible, you have to wonder why it's being applied with such zeal.
Most of the commentary I've read questioning the 20 mph scheme has focused precisely on these borderline cases, not the town centres or housing estates. And the whole narrative has been skewed towards these marginal cases rather than towards the benefits of the scheme in the majority of locations where no-one really questions the new limit, partly because you probably can't do more than 20 there most of the time anyway.
I agree also with Helvellyn on the desirability of the authorities making reasonable attempts to get it right from the beginning and not taking a "slap it on and we might reassess it later" approach. I feel this partly for the reason Helvellyn gives, that obvious and widespread deviations from common sense cause contempt for sensible regulation. But I also think authorities in general tend to forget that when they implement things like speed limits they are creating conditions that can result in criminal prosecution of and, if convicted, a criminal record for citizens. If you (generic) are going to do that, it is incumbent on you to be at least as conscientious in defining the rules sensibly as you expect everyone else to be in sticking to them.