I haven't seen one in person, but read several product reviews. The Traeger Flatrock and the new Weber griddle do have two clear advantages: (1) more uniform heating across the entire surface and (2) better low temperature control. I usually cook just for myself and rarely use the entire griddle surface. My Blackstone gets hot in the center and the edges are only useful for keeping the food warm, and that works for me. Also, turning one or more burners off is an easy way to achieve lower temperatures for me.
But I can see that these features would be attractive for those who use griddles to cook for a crowd. (Unlike my Blackstone, you can use the Traeger and Weber to make pancakes over the entire surface and they will be done at the same time!) Only they can decide if it is worth the extra cost.