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Would running decurion ever not be worth it in competitive play? What scenarios wouldnt you use it in or is it just a default?

Losing ObSec, especially in maelstrom missions, is really important. I think that a CAD supported by a destroyer cult, judicator battalion, or some canoptek harvests could be even more powerful than a decurion against certain armies that use msu ObSec, which decurions tend to struggle against. You can also use a CAD to field some powerful but less common lists like scarab farm.

However, you should note that the two Necron lists that made top 8 at LVO both used decurions. (I think it was LVO, please correct me if I am mistaken) The insane resiliency and versatility granted by the decurion and its component formations is extremely strong. I think that more often than not the decurion is the way to go, but I think it's possible to build and play an equally strong CAD, but it is more difficult.

Well, Decurion is generally the way to go. As said, you may struggle in some matchups in maelstrom missions, but the resiliency of the army balancies this out.

CAD > Decurion when you face gladius that will just roll around stealing your objectives. Resiliency means nothing when they don't need to kill you to win.

The Gladius is a different story.