According to the paperwork included in every smoke detector and CO detector
sold in the US, there is a disclaimer that states there is a high failure
rate among these products. In some cases up to 50%, but the jurisdiction
involved dictates if they have to state the actual failure rate or merely
say a "number" of these products can be expected to fail.
There are countless stories of these products going off after the FD has
responded and the fire is out.
They're a crap shoot.
In the event of an actual fire, if there is any warning, a good plan saves
more people than anything else. If harmful gases overcome people before
they respond, the story turns the other way. There are different kinds of
smoke and CO alerts out there, and their reliability is all over the map.