Is the miracle thaw safe?

It's certainly safe in terms of food contact. See How does a Miracle Thaw work? - it's really just a sheet of metal. It's not very much different from just putting your food on a baking sheet. Just be sure to clean it between uses as you would with anything else that touched raw meat.

However, you do still have to be careful about general food safety: it's not safe to leave meat in the danger zone (40 to 140°F) for more than a couple hours. The miracle thaw speeds things along, but it doesn't change that. So if it makes your meat thaw enough in half an hour, awesome. If it still takes 6 hours, you need to find another safe thawing method to use instead.

A thawing tray is basically a heat sink, a device that helps facilitate moving heat energy from one object to another. Metals are good at this, some more so than others. As Jefromi states, a simple baking sheet will do the same, though with less mass it will not do it as well. The more mass of the same type of metal, the more of a thawing aid it will be, and being less reflective such as the black finish they put on most thawing sheets often helps with the heat transfer.

As long as they are kept sanitary and the safe temperature rules are followed, they can improve food safety a bit by allowing the item to thaw more quickly so they do not stay in that danger zone for too long. But, if you do not get the item, especially meat, off the tray and cook it or store back below 40F promptly you are then exposing raw food to that danger zone needlessly, and I would call that an issue. If the item is too large to thaw in a reasonable time, the tray can then start to actually slow the thawing process. I have also known people to warm the trays. I would consider this a very bad idea, about equal to thawing in a heated pan, not particularly safe nor likely to give you the results you want. The general idea of using a heat sink is to gain some of the benefit of higher temperature thawing without actually using the higher temperature and its associated risks. I think the half hour Jefromi mentioned is likely a good guide. If it takes more than half an hour to thaw, then time to get the item back into a cooler environment, the tray is likely no longer helping the process.