Giratina O had even weaker 120/120 Offenses and base 90 speed, but many people still got him to work including myself (not to mention he is the best spinblocker in the game still). I bet Kyurem might have a specific function, since he is probably now the best Hail abuser in the game and can be used in hail teams with Abamosnow. He is definitely not outclassed by Rayquaza due to his higher HP and neutrality to ice moves. I could even see a bulky spread for this, since 125/90/90 is kind of above average, and is very similar to Giratina O. I can see this becoming like Giratina O, basically an underrated pokemon that became a top threat with people investigating it more. Despite his limited movepool, Kyurem does have Ice and Dragon stab, which is very effective in ubers. Yes, he has many flaws, but some of the benefits outweigh the negatives.
Giratina-O resists Bug, Electric, Fire, Grass, Poison, and Water. It is immune to Fighting, Ground, and Normal, as well as Spikes + Toxic Spikes. Its STAB moves hit most of Ubers for super effective damage. This guy resists Electric, Grass, and Water. It isn't immune to anything. Dragon + Ice offers redundant coverage. While he'll certainly have a niche, comparing him to Giratina-O is insane, to be frank.