Easy. You're 3x more productive for a while and then you burn yourself out.

Or lose control of the codebase, which you no longer understand after weeks of vibing (since we can only think and accumulate knowledge at 1x).

Sometimes the easy way out is throwing a week of generated code away and starting over.

So that 3x doesn't come for free at all, besides API costs, there's the cost of quickly accumulating tech debt which you have to pay if this is a long term project.

For prototypes, it's still amazing.