Even as an enabler, Sustrans are the route numbering authority, aren't they? They're responsible for the travesty that is cyclists being unable to count on their red numbers as signifying a useful all-weather cycle route. Even within the same highway authority area (Norfolk), Notional Route 1 varies between
wide/smooth tarmac and
near-unrideable sand,
dangerous gravel descents ending in barriers and
narrow dirt track.
I feel they should be stricter about delisting sections which do not conform to their design manual, but I think much of the current substandard network was accepted by them in a Lottery-funded push to have a route within X miles of Y% of the population by the year 2000 and falling back below that carries some reputational risk (if not having to refund the £43.5m lottery grant).
But why would they? As a self-perpetuating private charity, they're basically not accountable to cyclists, as long as enough people are willing to donate money.
There's also their key involvement as a trusted partner of central government enabling the waste of cycling budgets on dross such as the Bedford Turbogate but they did at least ask cyclist-led organisations to get involved then (and none covered themselves/ourselves with glory in that example IMO).
I've posted about this before, but it's worth mentioning again.
NCR66 passes within a mile or so of my house, but there is nothing on the Sustrans website* to tell you what sort of surface it has, and as it takes you from the heart of a major city into some great countryside it should be a jewel in the network.
But turn up on an MTB and you'll spend an age slogging along on roads or shared paths and having no fun. Turn up on a road bike and you'll sink to the axles in mud, if you haven't already knackered your bike on the stony, rutted bridleway sections.
And that's all assuming you can follow the route as signage is diabolical in far too many places leaving you with no idea where you are meant to go. At one point the route takes you up a dead end street that ends with a high kerb - you have to dismount, lift your bike up onto the footpath and try and spot where the route goes next...
EDIT: to add this picture showing what I mean -
You have to go up the path to the bollards just visible, where there is a dropped kerb with a splash of red tarmac on it which then takes you onto a mini-roundabout and is again unsigned leaving you to guess where to go again.
This is the route from the other side looking down the slope:
At least here there is a clue with the badly eroded red tarmac, but look at the bollard slap in the middle of the 'cycle lane' and the ambiguous sign that could easily send you along the main road.
Other routes locally (655? at Wetherby and whatever the Solar Cycle at York is designated as) are former railway lines, yet the tarmac is rutted by tree roots, frequently covered in debris and oddly narrow for the available space with just enough room for two bikes to pass.
* - I'm aware you can find some details on some route surfaces elsewhere, but it's hardly easy to find when the Sustrans website should be doing it