About 80% rewrite: it is poor abstraction problem rather then vendor lock-in one

Abstracting over your database only makes sense if you're writing something that's lowest common denominator. If your application runs equally well on MongoDB and something else you either are running poorly on both or you are spending a lot of resources on writing your abstraction layer.

Yeah, that contributed, but it wasn't any worse than most other CRUD apps I've seen.

"you know that you could throw everything away" To be fair, that's like 99% of all hackathons I've been to