All of the Cover Legendaries aside from Lugia/Ho-oh and maybe Xerneas/Yveltal (they had to wait until after XY to get that) have gotten notable backlash at their initial reveals, so it's not too surprising. As time goes on the vocal hate will calm down and people will be calmer in voicing their criticisms, though it will partially depend on how they're implemented.
Personally, while they're not my favorite duo (Reshiram and Zekrom still hold that title), I personally like both of them. Their designs are pretty cool and I like how some of their extra details have a use in their concept (ex. Zacian's wings becoming a part of its sword), and the brothers in arms feel that the two have at the moment is pretty neat. Hopefully their story and lore makes good use of their potential though since I'd hate for them to feel shoehorned in like what happened with XY.