this is the key part. their niches actually aren't that comparable. the advantages of special based lo kyub vs hydrei are pretty trivial aside from resoundingly threatening hippo (important in the context of dragmag where the set is most commonly seen) and checking lo starmie. hydrei is almost always better in the special/mix department because of its more unforgiving coverage and higher splashability from being less hazard prone and adding on a fire resist that can be helpful against tran especially if you go full special. furthermore, special hydrei's ability to switch headlong into mew/jellicent/other waters and get real dark with them is another valuable feature for the sand offenses in which it thrives that kyub doesn't offer. that one needed emphasis.
now hydrei's niche held against the full scope of kyub, including the physical sets (mainly cb) where it distinguishes itself from the entirety of ou, is a more difficult analysis, but i'd be inclined to designate them both A+ with a slight edge in hydrei's favor. for me, it simply boils down to hydrei's aforementioned splashability filtering into the consistency and ease of play of its slot, factors that i think s/upper a should epitomize. layman's version: it switches into more attacks, it switches more freely into hazards, it therefore fires off more attacks while being almost as damaging as kyub's highest break potential sets, etc etc.
if u really wanna grasp unforgiving coverage tho i suggest u load up that cb N.I.G.G.A. with about two moveslots sometime. it's a higher opportunity cost selection but a higher reward for sure to punish people who like to be able to switch.
