IDK if anyone has posted this before, but I watched a random Youtube video that explained why the BBH is so OP in this Nightreign.

Basically, they added a special stagger animation to every single enemy in the game that is activated when certain Ultimates are used (wylder, raider, guardian, ironeye etc). HOWEVER, they just forgot to add this animation to the BBH.

So he can't ever get staggered unless you get a full stance break on him (which opens him up for crits). Ulting him as a Wylder for example will never stagger him because he doesn't have that special stagger animation.