... but if a car battery isn't producing 12V consistantly, it a problem with the car (or battery needs to be replaced)... so there that too...
The problem with typical lead-acid batteries is that they perform worse at low temperatures ... even if they are in excellent conditions. And during those few seconds it takes to crank-up the engine, the battery voltage can drop (well) below 12V.
That is actually a big problem for any serious electronic equipment ... we've been using some professional measurement equipment for measurement of PLMN signal (GSM, UMTS, LTE) and the equipment actually contained own UPS-like hardware (internal battery that could power the whole measurement setup for a few minutes). And yet, equipment vendor stated that vehicle's start-stop system should be disabled while using the measurement equipment exactly due to voltage dips that occur during starting the engine and which might disturb the electronics.
And no, 75% conversion efficiency is not good when it comes to 24/7 operation off a battery. Typical car battery has, say, 60Ah capacity (smaller cars have even smaller battery). With current draw of 1A (that's 12W at 12V), the exemplary battery could sustain such device for 60 hours - two and half days which cover a quiet weekend. If we throw in a 75%-efficient conversion device, we effectively decrease the autonomy time to 3/4 of previously calculated. And I even didn't mention that deep discharging of a typical lead-acid car battery causes damage to that battery and should be avoided.
And the sad fact is that a battery with low remaining charge (say below 20%) can have hard time to start the engine. So a setup which includes 24/7 off-the-battery operation should at least include an under-voltage protecting device ... which disconnects its load when supply voltage drops below certain threshold (can even be 12V, covering lower bound of operating voltage of LtAP) and re-connects when voltage rises above another threshold. Such under-voltage devices come pretty cheap (
example device on aliexpress can be found for a few bucks), but might consume some power themselves (the linked device consumes some 50mA at 12V) causing car battery to drain even faster.