I thought the Raid was doomed to fail the moment back in chapter 920, when Kinemon stated that even with gathering the 5000+ men he "doesn't want an open war" and this is designed to be a precision raid to take out the heads of the enemy leaders, namely Orochi and Kaidou.
Some argue that too much has been invested for the raid to fail. In-verse the flashbacks shows the tragedy of Oden and Wano and the burning desire of the Scabbards and the people for Onigashima to succeed. The Scabbards march on under the sun, determined to avenge their lord. The Samurai scupper their ships, intent on never leaving till they succeed or die trying. Oda shows us in 70 chapers of Wano just what the stakes are. The people pin all their hopes, 20 years of suffering, Yasuie's sacrifice. All of this, and more. The Raid has to succeed. It just has to.
Couple that with the SH's growing in power. Luffy training for a week. Kidd wanting vengeance, Law scheming. Hawkins potentially wavering (if he's still alive). And seeing how divided the loyalties of the Beast Pirates are along with Drake's obvious treachery. Speaking of which there is Big Mom and her crew around - wild cards who are no strangers to treachery and taking advantage of a situation. Especially if it could mean Big Mom taking down a rival. The Beast Pirates are rife for picking.
So yes, based on everything we've seen, it should be a slam dunk that the Raid will succeed, surely? Oda won't undo all the work put in by the good guys right? They cannot have setback, not this deep in the arc. Some bemoan the prospect of the Wano Arcbeing extended even further if the Raid does fail.
But here's the thing. Back before the Timeskip we had a similar situation where everything was built-up to save Ace. Lufffy went through literal hell from the end of Amazon Lily onwards through Impel Down. Mr. Bon Clay (before Oda undid it all...le sigh) stayed behind, apparently sarificing himself so Luffy could save his brother. Back then, many thought Bon Clay's heroic sacrifice was all in vain.
The Whitebeard Pirates failed to save Ace. Whitebeard failed to save Ace and died as well. Luffy gathered allies and freed Ace, and still failed.
(Interestingly Wano has been an inverse of Marineford. Back then it was to 'save Ace'. Now, it's to 'kill/take the head of Kaidou/Orochi.')
That's why I believe the Raid failing was always meant to be. We get Narrator speak heavily implying the war will happen ('seas will split yaddda yadda'). Meaning the Raid will on some level, fail. Then everything goes out of control. Maybe a better way to put it though is that the Raid actually succeeds. Until the unforeseeable happens.
Similar to how it went down during the Tea Party. The plan worked, they destroyed the picture of Mother Carmel. But, no-one, fans or characters alike could have foreseen that Big Mom's scream was strong enough to destroy the VFX rockets launched at her while she was at her most vulnerable.
If the plan worked then Kaidou and Orochi get killed off and its all nicely contained on Onigashima. Right...