From what I can tell, defrosting trays are only of any practical use when defrosting very thin cuts of meat e.g. burgers or fish. Anything more dense or substantial (e.g. a ready meal, a small frozen chicken or a lasagne etc.) will require more than 2 hours on the tray at room temperature before it has fully defrosted, which defeats the whole object.
Are there any other scenarios where these devices are clearly a threat to food safety?
There is always a threat to food safety when thawing above refrigerator temperatures. There is nothing about a sheet of metal that changes those safety considerations. By the way, you can achieve the same result with a stainless or cast iron pan, and therefore don't have to purchase a "defrosting tray." This works by conducting heat away from the product more quickly, but, as you point out, in some cases it might not be quick enough. The guidelines in the linked, "defrosting on counter", question should be sufficient to address the issues. The threat to food safety occurs when it takes too long to defrost your product.
