It shouldn't affect safety. Otherwise also an RT failure, a blocked frequency etc etc would also affect safety, but they shouldn't, if safe instructions are issued.
If you point 2 aircraft at each other planning on turning them later and then the PA announcement goes off, it's not the pilot's fault
But it does and it doesn't matter what frequency it happens on either. I've had several over the years and have on occasion had to make several calls to other sectors to recoordinate at different levels due to not being able to get in and issue a final instruction to meet a standing agreement. We all rely on our colleagues to put planes where we expect them, miss one and the pattern is very full very quickly and those windows can become instantly useful.....
There isn't any way of avoiding it other than making the method of talking to the cabin vastly different and harder than talking to ATC, but I doubt a cockpit would welcome that either.
As for a backup frequency, you could always talk to the D & D cell on 121.5 in the UK, but they wouldn't be able to do much either. Best sit and look at TCAS for the duration........