Pokemon has elements of chance, that's how the game is. You can sway the odds in your favour, however, 30% sounds infeasible, but consider, as many have said before, that it's a 51% chance to KO at least one Pokemon over the use of two turns. Why would you attempt to use Sheer Cold on something like a DD Rayquaza? There's a niche use for Sheer Cold on Kyogre, and that Pokemon getting in for free? They risked a 30% chance of dying, 51% if they're slower. Just because it could go either way doesn't mean it's the definition of 'non-competitive', many a time has a game come down to a Focus Blast miss, a Scald burn, etc. Using Sheer Cold against offensive teams is a high risk, high reward strategy, but the risk is much higher, meaning you'll lose most of your games. Losing to a bunch of sequential Sheer Colds hitting may seem 'unfair' or whatever but there is nothing to differentiate that from 'competitively' losing to an Arceus crit or any other random roll of chance. Also, more often than not there is a workaround to most things - Mamoswine can check as many Rayquaza as it wants, Yache Berry Rayquaza lacks the power to get through Ice Beam Arceus; almost no strategies are foolproof (and if there are some I'd like to know) and the metagame can usually adapt.