These types of devices - even commercial ones - are absolutely NOT designed to raise the temp of foods ... only to hold it at a safe temp once it gets there.
Reheat ALL food in a proper cooking appliance before you put it into a holding device.
Period.
No exceptions.
Now, to be sure, these things will raise the temp of even a frozen item placed in them. The issue is the amount of time that takes. Almost all food can only stay in the 'danger' zone (between 45 and 140 degs) for no more than two hours. In actual fact, this can be stretched to three hours, but at that amount of time, and assuming the food was completely bug-free to start with, you're ***right*** up against a dangerously high plate count for bacteria.
These holding warmers would take many hours to raise an item through the danger zone to the safe zone at the hot side. And the denser the food (like a casserole) the longer the time. As I said, this advice applies to both commercial and home devices.
As a side note, the unit here is 1200 watts. And that's perfectly fine for general home use and will hold food safely (assuming the food does not protrude above the rim of the pan). Commercial units that look similar are in the 1800 watt range - and even they are incapable of safely reheating food.