..... what is the likelyhood of the door not opening and why would it not?
Failure of the power supply or a fail-safe electromagnetic release, in such a way that the door is unable to be used to escape, is unlikely. Or, at least, in any way that the inclusion of an EDR switch (green box) would have any benefit.
The Request-To-Exit switch can be a real problem. If it is wired as a simple push-to-break switch directly to the power energising the lock, then you have the likelihood of the switch itself failing in some way affecting the door lock releasing. I don't believe there is any minimum manufacturing quality standard for a RTE switch. If it is wired, instead, to some other door control equipment (such as keypad and/or proximity reader type control) it may NOT be wired in a fail-safe manner. (it all depends on the particular set-up of the 'other door control equipment' wiring needs). Even if it is wired as a simple push-to-break switch then really it should be wired to break both power feed legs (like an EDR switch and for the same reasons) and, it might be argued, that a simple unobtrusive RTE switch may not be easily identifiable in a panic situation.
The link to the fire alarm to release the door is fraught with potential problems. BS7273-4 has tried to address these and has created a plethora of recommendations which are difficult to understand and, in my opinion, creating a cost implication that far outweighs the benefits when an EDR
is correctly installed.
The green-box EDR is, in my opinion, much more important than a link to the fire alarm system, which can be difficult or expensive to achieve correctly.
In my opinion, not only should correctly installed green box EDRs always be used, they should maybe be made mechanically more reliable (if possible) and easier to identify as to their purpose in an emergency. They are probably the most important piece of equipment in the whole electromagnetic door release set-up.
