The worst scenario is when electronic displays aren't working but TM insists that reservations are valid. I've had this a number of times recently when I'm moved from seat to seat with no clue which are available or not. Not pleasant on a long journey.
In my opinion if displays are not working then all reservations should be void. End of story.
Personally I can see the arguments for either policy, particularly given that more people reserve these days than used to. So I'm pretty neutral on what it is - indeed, I tend these days towards a view that reservations should be unmarked (just marking fixed reserved/unreserved areas) which would mean they could be done at any time by far more sales channels, reducing the effect of the Euston scrum and its brethren.
But what
is needed, and it really utterly beggars belief that not a single TOC has done it yet as it seems a glaringly obvious thing to do to avoid conflict, is clear signage, visible from every seat and the aisle, in all coaches stating:
1. What that policy is (i.e. if the reservation screens are not working/there is dispute if they are showing Available wrongly, that they either apply or they do not)
2. Which is the unreserved coach or area and which way it is (or if you are in it)
Guards announcing doesn't work, as they have station duties to do so they can't announce it at the point when it actually matters (while people are boarding).