Ken, I understand these ventless logs are far safer than the kerosene heaters. I would not have any trouble using one in a fireplace that was burning a few hours at a time. It is noteable however that some of the States with the most active public health departments are the ones that make it illegal to use them (Alaska, California, Massachusetts, Minnesota, Montana, New York, Utah, and Colorado ). Consumer Reports, for what it's worth, also discourages them. I think the point is not that the modern ventless systems are likely to kill you overnight (as you mentioned the oxygen monitors would work to prevent that) but rather the unknown hazards of long term exposure to the minor products of combustion. There simply aren't any long term studies that show us these things are safe. You could probably make that argument about other commonly used appliances/habits etc but in this case there is both plausible harm (air pollutants) and a readily available alternative (B vent or direct vent gas heaters).
The immediate overnight safety I am not so worried about. It is the longer term exposure that I find troubling. (I know the quote from Canada did mention that overnight death risk but that is not my main concern). That is even more a concern these days as we learn the importance of "tightening" our houses thereby reducing fresh air exchange. That is particularly a concern for anyone with asthma or other lung disease.
Another concern is that these devices have been around only for 15 years or so. We don't know how long they will perform up to specifications and I doubt seriously that anyone does professional preventive maintenance on something like a ventless fireplace log. Nevertheless, it is certain that heat, moisture, expansion, dust, direct damage etc will eventually cause the heaters to fail in some way or to simply spew out more pollutants. Direct vented and B-vented gas appliances have a fall back safety mechanism that ventless do not have.
Again, I wouldn't have trouble with a ventless design for an occasionally used fireplace but I do have concerns for these devices as primary heating units especially in northern climates where that means full time use for months at a time.
Here is a good short summary of the argument:
Ventless Gas Stoves article by Woodstock Soapstone Co.
New gas code(international ) states that space heaters can no longer be used as only source of heat.