There are operational tailless aircrafts such as B-2 spirit, X-45.
It seems yaw stability/control at slow speed is achieved aerodynamically, using either split elevons (B-2) or adjacent elevons deflected in opposite directions (X-45) Combined with negative wing tip twist angle, both solutions increase drag / relative swept angle, and by the way, yaw stability. Also note both examples have a very small vertical surface ahead of CoG.
OTOH, looking at this picture of a F/A-XX Boeing concept, it seems vertical surface ahead of CoG is quite significant, and forward swept trailing edge is in contradiction with aerodynamic stabilisation using split elevons.
One might think this isn't such a big issue since this concept might use permanent vector thrust adjustments on yaw axis. Question is : could it survive the loss of one engine? And could it even glide with the loss of two engines? ( B-2 can, X-45 can )