The downside is that the business people don't understand why you can't just put their app in production as is. And then there's a lot of pressure to make it happen quickly "because we can use AI to move fast". This is going to come down to healthy organization dynamics, and hopefully represents a learning opportunity for leadership.
The upside is that the idea has already been proven out much more thoroughly than a sketch on a napkin. Claude has already prompted them about edge cases and design decisions. It's very likely that at some point they had to explicitly tell Claude "don't worry about that and just make an assumption", or "I actually don't like that interaction after trying it a few times, can you do a differently". They had to write out much of their idea in clear and direct language in order to prompt the AI. They've probably been playing with their own toy because it's fun to play with toys you summoned from thin air, so they've had a chance to discover the experience and refine their own preferences.
It's probably a net negative right now because the "ship it what's the problem team" pressure is intense and stupid and demoralizing and miserable to work under. But I think it will stabilize and it might be a net positive in future projects.