You still have absolute certainty. Read or write x amount of data and it will use exactly y R/WCU.
It then just becomes a modeling problem allowing you to determine your costs upfront during design. That’s one of the most powerful features of the truly serverless products in AWS in my opinion.
Absolute certainty is challenging with a cost-based optimizer in the mix. DDB doesn't face this challenge. Although, cost for some query patterns in DDB would shift into your application layer - so you may not have exactly the cost certainty you imagine?
Would you be willing to pay more for certainty? E.g. rent the full server at peak + 20% and run at 15% utilization some of the time? Provisioned capacity or pre-committed spend seem like reasonable, but perhaps more costly, ways to get certainty.