l'd imagine the key question is how often such an incident occurs, plus how it is managed. For example a loco where the crew spot the symptoms in time so it can be pulled off the main line / directed into a platform where it will not cause issues to the ordinary train service (as occurred in the cases with Scotsman at Peterborough) will cause far less friction with NR than a loco that ends up sitting down blocking the line and delaying a whole heap of service trains. However a loco that repeatedly ends up having problems and causing disruption is likely to re-inforce any prejudices that steam shouldn't be allowed out to play on NR infrastructure regardless of what the fault may be. Similarly if Enthusiasts / members of the public cannot be trusted to stay outside the boundary fences then it creates perfect ammunition for the "anti-steam" faction. Thus if a loco is subjected to a rigorous maintenance procedure, the crew know what they are doing - and particularly what to look out for in terms of regular inspections during the trip, plus enthusiasts / the public behave themselves when viewing such trine, then the "anti-steam" brigade (and yes they do exist) won't have any ammunition.