The Legends, the Ham'r, and many of the other "Instant Death from Above" new cartridges designed to be shoe-horned into an AR share many of the same problems--mostly due to the magazine limit of a COL of 2.3" or less. Then it becomes a war of attrition between case propellant capacity and inherent ballistic efficiency of the bullet's design. That limits the selection and performance of bullets to experiment with. That said, if the 400 was out at the same time as the 350 legend I would never have even given the slightest thought to the 350 and would have immediately chosen the 400.
The new 300 Ham'r I put together may possibly be even more ridiculous than the 350 Legend--but at least I have less to worry about the cartridge blowing up with the Ham'r.