No it is not, with a chest freezer yes it will be likely OK for 12 hours, but with up right frost free it depends on where it was on the cycle when the power failed. .... So length of time it can be without power depends where it was on the defrost cycle, my freezers show the maximum temperature reached during a power cut until the door is opened, when it returns to current temperature.
I think you are probably being unnecessarily conservative/pessimistic, particularly in relation to domestic freezers.
In terms of chest freezers, one often sees "up to 48 hours" mentioned and my personal experience is that (with the lid closed, and freezer 'nearly full'), the contents remain 'well frozen' for well over 24 hours.
As for 'frost free' upright ones, as I've said, I've never had one. However, I had assumed that the 'no cooling' part of the defrost cycle was relatively brief, so that (provided door remained closed) the temperature of the cavity (and contents) would rise little during that period. Is that not the case? If not, how high does the cavity temp go during the 'no cooling' part of the cycle?
The other thing is that, even if temps do rise, the advice we are given is ultra-cautious. The reality is that even 'total thawing' (which is unlikley to happen) followed by re-freezing is extremely unlikely to cause any 'health' problems, particularly in the case of things which are going to be 'properly cooked' after removal from the freezer.
To put this in context, when I manually defrost one of my (large) chest freezers, I leave most of the contents lying around in crates (in cellar, garage or wherever) in 'ambient temp surroundings' for probably at least 3-4 hours in many cases. It's only things like 'ice lollies' or items which will ultimately be eaten without any further cooking, that I bother to put into another freezer.
The amount of food will also affect how long it can go without power...
Very much so, and many people don't seem to realise that. The more 'space' (air) there is in a freezer, the more rapidly will the temp rise when there is no active cooling. However, that's rarely an issue with our freezers, which always seem to be 'ridiculously full'!
... but I know with shop chest freezers they have alarms to alert the manager at home if they fail over night, I know my son-in-law is a shop manager and has needed to go out to sort freezers when alarm has gone off, shop opens around 8 am closes around 10 pm so only 10 hours with no one there, if the freezer could last 10 hours then there would be no point in having the alarms. Or paying my son-in-law to drive 20 miles to sort it should one fail.
That's a very different world from the domestic environment. Freezers in shops don't usually have significant insulation in the doors/lids - often just sliding perspex/glass panels, and some of the chest ones don't have lids at all. Furthermore, I suspect that they may be constrained by regulations that one requires them to throw away food if the temp of the freezer has risen to some arbitrary number for even a very short period of time - although, as above, I suspect that any true risk' is usually very minimal.
Kind Regards, John