When you set up only to get KOed by ditto.
Yes, it is completely correct. The viability of Ditto depends upon how much the ladder is HO i.e. Hyper Offense. Talking about the higher ladder, most teams are comprised of Bulky Offense or stall, no one uses complete HO to protect yourself from Ditto-ers. That's why Ditto is in -A, due to the higher ladder people tending to run important checks to their same team, e.g. A Zacian-Crowned appreciates support from Primal-Groudon, Dusk-Mane Necrozma so that they can revenge kill Ditto and protect themselves from Ditto, since it can easily sweep away teams.
Equipped with a Choice Scarf, Ditto is always able to outspeed any foe that it has copied unless they also hold a Choice Scarf or Ditto is undergoing Dynamax. Its ability Imposter is the reason it excels as a revenge killer. By causing it to copy all of the opponent's stats except HP, it is able to beat most sweepers such as Zacian-C, Mega Rayquaza, and non-Dynamaxed Xerneas by always hitting them first with boosted moves thanks to its Choice Scarf.
Ditto is one of the the best revenge killers in AG. It can make great use of Dynamax, which lets it snowball on the opposing team after having copied a boosted sweeper like Xerneas, Necrozma-DM, or Mewtwo. It is ideally sent out on the field after a teammate has fainted: Ditto can then revenge kill the foe or, if the opportunity allows it to, Dynamax to break its Choice lock and blow through the opponent's team thanks to augmented moves and doubled HP. Once its Dynamax is over, it can potentially continue to sweep with its Choice Scarf. It can also immediately copy Zacian-C and get an extra boost from Intrepid Sword that lets it break past traditional counterplay to Zacian-C such as defensive Necrozma-DM and Primal Groudon with Close Combat. Ditto is self-sufficient and fits on every playstyle, but it is typically of great value on balance teams that would otherwise be overwhelmed by the myriad of sweepers that can resort to Dynamax to bypass defensive and offensive counterplay alike.
How it gets countered:
Yveltal cannot be beaten with Ditto during any of the three Dynamax turns because it has a higher HP stat and a Life Orb, and even afterwards, Ditto can't revenge kill it without Dynamax unless Yveltal is seriously worn down. Xerneas also needs to be seriously weakened or unable to Dynamax before attempting to revenge kill it with Ditto. Ditto can struggle against defensive teams with Pokemon like Zygarde-C, Necrozma-DM, and Ho-Oh that it can do little to.
This proves that how much important is it to waste your opponent's dynamax to make Ditto stand "to some extent", since it easily gets beaten by Yveltal. Also, since Scarf is disabled when Dynamaxed, it means you're at a huge risk of loss. Some players are even running Choice Scarf e.g. Calyrex-Shadow to stand a chance against Ditto. Plus, what happens if they switch to Ditto? Then you're locked to Astral barrage and have two choices: Dynamax and hit it hard, or switch. If running HO team, then opposing Ditto will harm your team, and as already told, Scarf is disabled in Dynamaxed, means they're Calyrex can beat you. All this demerits make Ditto -A.
Hope this helps!