Although there is no logic in feeling this way, I just do not like rebated rims.
There is logic to it - rebated rims can sometimes cause feeding difficulties. The .350 Legend has a rebated rim, because it's essentially a 9x19 (tapered case) stretched out to 9x43, with a 5.56 rim instead of the 9x19 rim, so it can function with existing 5.56 / .223 / .300 Blackout bolts. It appears to have worked out well enough, that the same approach is being used for the .400 Legend - it is a tapered case like the .350, and NOT a straight 6.8 SPC case as suggested above (the base diameter is larger than the 6.8). And because they wanted to use an existing bolt design (6.8SPC / .224 Valkyrie), it's got a rebated rim.
I'm not sure if they're designing these with tapered cases to increase case capacity, aid feeding & extraction, and/or possibly head spacing, or if it's just to sell brass. Another issue for some folks is the need to lube cases before resizing; if these were true "straight wall" cases, it would be easier to make carbide sizing dies that wouldn't need lube.
But, if it ends up feeding reliably, it could still be a good cartridge (and fun to reload)!