Ekiller on sun offense has proven to be insanely difficult to stop for even prepared teams. And even so, many teams are very reliant on mons such as Giratina-O to stop it, which has a 21.25 % chance to outright lose due to a crit or dtail miss. Checks are easy to fit on teams but for most it takes rather little chip damage to get by some and barely anything outright OHKOs it (even for offensive teams). Offensive teams will always risk getting swept and I don't think there are many mons that obliges you to keep some of your mons at certain percents just because of the chance your opponent might have it.
I personally find Fightceus or Ghostceus more useful (albeit obviously not as threatening in practice, unless Ghostceus hits all Focus Blasts), and obviously the third most viable Arceus-forme doesn't belong in S-rank rank.
I think Edgar brings up most of the relevant points, tho I disagree with the Ghostceus stuff. On a hazardless field, Ghostceus only 2hkos Ekiller about 15 % of the time and that's excluding SForce or Sclaw crits (and jolly, which funnily enough has a slightly lower chance to beat Ghostceus). If SR is up, which is to be expected in BW2, Ekiller still wins 51 % of the time excluding a favorable Sclaw crit (jolly is much better on a hazardless field, halving Ghostceus' odds). So yea being slower than Ghostceus isn't necessarily always bad.
With Ekiller I don't think there is much need to overexaggerate what it can do, nor do I think it's constructive to downtalk what it fails at. All in all it's BW2s most threatening sweeper but unlike in ORAS that doesn't mean that it's the best mon in the game.