I don't think it is, or probably ever will be. There are always 1000 details you have to get right.
I'm suspicious of projects described by appending that word "out" after "build." It implies the speaker is underestimating the importance of execution. In the best projects, execution is not merely important, but even flows back into design.
If the idea is extremely simple and your team has talent then you could do a better job with someones idea than they could.
I think build out is perfectly valid way to explain building more functionality on a idea.
google didn't build ad functionality into from the start their search engine it was built out afterwards. I come from a family of builders so this makes sense to me.
I also don't agree with being secretive, ideas are a dime a dozen so no need not to share.