Uncompetitive means, basically, that player skill has been taken out of the equation, or severely reduced in importance. In the case of Heliolisk, pretty much any team can take it out, but it requires prediction to do so (attack when they attack, don't attack when they use Electrify) if you aren't faster than Heliolisk, don't have priority, have no Speed boosting, and don't have Mold Breaker, with the consequences of a mis-predict being strongly weighted toward the Heliolisk player.
If you mispredict, the two possible consequences are A: they take no damage and get a +1, and after enough of those can sweep a team lacking priority and/or Pokemon faster than Heliolisk or B: they dish out damage to you while you switch, use a self-targeting move, or otherwise do nothing targeting Heliolisk. If they mispredict, the two possible consequences are A: they dish out damage to you but take damage in turn (In a lot of cases, this will be a mutual 2HKO and Heliolisk can finish the target anyway) or B: they use Electrify and nothing happens but 1 out of 32 PP is wasted.
So the best case scenario for the non-Heliolisk Pokemon is pretty bad, if it isn't something like "use Swords Dance and then successfully predict an attack to net the OHKO", which is hard, and screwing it up can result in you fainting without ever touching Heliolisk anyway, Swords Dance or no.
This is a long series of 50/50 type predictions that favor Heliolisk, with even things going wrong for it tending to amount to "Heliolisk KOs a Pokemon before it goes down, having mauled another Pokemon first". It's not broken, not in the usual sense, as it requires a certain amount of luck -prediction, rather than actual dice- to become an abomination, but said luck favors it in terms of how the payoff is weighted.
