It causes games to be decided on luck. It truly does. Whether you hit your OHKO, something is going to die immediately; and the Sturdy Pokemon can be safely removed with Kyogre’s Scald, which was proven to be by far the most deadly OHKO user. If you get your OHKO move hit the right Pokemon, or you KO a Pokemon that already occupied boosts, you have an extremely solid way of winning. 30% may not be enough, but there’s a less chance to hit with a critical hit. I’ll take Pocket’s explanantion here; let’s say you’re facing a +6 Arceus-Steel and you have Sheer Cold/Scald Kyogre. A Scald critical hit doesnt’ even KO in rain, whereas Sheer Cold hitting, which has a better chance to happen, simply KOs Arceus, probably saving you the match. The usage of OHKO clause, makes games based on your chances to hit the move--makes games based on luck, which is something all of us do not want.
Lifting OHKO clause throws the metagame off balance. Stall is still viable, it really is. However, Stall relies on every single member of your team to cover a wide array of threats, and getting one of those members removed to a 30% chance on a single turn, would simply make Offense the preferred playstyle, as it is much more flexible. Offense also enjoys the addition of OHKO clause more than Stall does, as now you can simply eliminate a Pokemon (guaranteed you hit) and sweep with the Pokemon that was stopped by the one you just eliminated. If a member of an Offensive team is lost, no worries, the team can attempt a sweep with someone else. OHKO clause created a situation where Offense as the upper hand over Stall, which throws the Ubers metagame to its edge.
High Risk, Higher Reward. Sure, you have 70% chance to miss the move, but you still have 30% chance to immediately KO the Pokemon that is hit by it. If you use it twice in a row, you have 51% to hit - chances are in your favor. If you’re willing to take the risk, there’s quite a chance that it will make out for you. Outright KOing a Pokemon is the definition of broken by me, not to mention that OHKO clause does not require any thinking: you click the move, and if it hits, something dies; if it doesn’t, you can either try again or switch to a proper counter. Nothing impressive.