It's not unusual for me to be waved through the barriers when a ticket doesn't work. I'll concede it's possible that an experienced attendant may be able to determine validity from a very brief glance at a distance, but I suspect that on some occasions it's because I don't look like a stereotypical fare evader. I haven't kept a running total, but I'd say that it's a roughly 50-50 split between thorough scrutiny and (at most) a cursory glance.
Gateline assistants aren't given much in the way of ticket training, especially if you're talking about LU gateline assistants and National Rail tickets. I know several LU staff who have experience of working on gatelines (before gaining promotions) and they learnt far more from this forum than their training covered.
Is it rejected by Rail Delivery Group out of hand, or just considered Too Difficult for the Train Operating Companies whose machines only do tickets from the station they are at?
Is it practicable for the machines at Paddington to offer a "from another station" option, and also decide for themselves between (a) Paddington National Rail, and (b) Paddington platform 16 = London U1 ?
To be fair, something does need to be done about Crossrail.
However the plan does appear to be to restrict tickets. For example Richmond to London Terminals is valid into London Euston and London St Pancras. The former is officially recognised and undisputed, the latter (despite previously being a direct service) is not officially recognised as intended to be valid by RDG, for reasons that are not apparent, but you can get tickets with itineraries and it is valid.
But head a bit further west, say to Twickenham, and while there are good people within RDG who agree that Twickenham to Euston is, and should be, valid, that is not a universal view. You can currently get itineraries and tickets are valid, but the intention of some of the 'powers that be' is indeed to re-route the tickets in a way that prevents their use into Euston.
In these examples it is nonsensical for RDG to deny us our rights; paper tickets are very expensive anyway and people can make those journeys on Oyster. But they seem hell bent on removing passenger rights and flexibility.
They want to scrap the Routeing Guide and replace it with something more restrictive.
It's a war on passengers basically.