I don't use it to thaw things out, instead I slow cook food overnight (technically over day for me) and pack that. It should be fine if it's already cooked, just gets cooked some more. Add lots of water or it'll dry out already cooked food pretty fast if you don't. Frozen/precooked food should be safer than raw food if temperature on it didn't kill the bacteria. But I toss in a piece of meat, potatoes, tomatoes, etc before I sleep and it's ready in the morning for breakfast/lunch.
A rice cooker on a timer works great too :) So does a USB powered coffee warmer/hot plate, I use one at home for keeping my tea hot instead of coffee.
but if it's just a matter of heating up the food, why can't you use a microwave in the morning then packing it away hot? Microwave is going to be able to heat food to the same temperature as a slow cooker for the intent of warming up a frozen meal.